fix(wrap/serena): install Serena from the serena-agent PyPI wheel, not the git source

## Description

`headroom/mcp_registry/install.py` (`build_serena_spec`) and the
wrap-time Serena pre-index in `headroom/cli/wrap.py` both ran:

```
uvx --from git+https://github.com/oraios/serena serena ...
```

The git source forces a from-source build. On proot-based filesystems
(Termux + proot-distro on Android, some restricted Linux) `uv` cannot
hardlink build dependencies into a fresh build venv, so the build fails
immediately and Serena's MCP server fails to start on every `headroom
wrap codex` launch:

```
× Failed to download and build `serena-agent @ git+https://github.com/oraios/serena@<commit>`
╰─▶ failed to hardlink file ... Operation not permitted (os error 1)
```

Setting `UV_LINK_MODE=copy` fixes it in an interactive shell, but Codex
strips most env vars from the MCP subprocesses it spawns, so that
workaround does not reliably reach Serena's launch.

Serena publishes the official `serena-agent` package to PyPI with
prebuilt wheels, and it exposes the same `serena` console script
(`serena = "serena.cli:top_level"` in the project's `pyproject.toml`),
so `uvx --from serena-agent serena ...` runs the identical command
without a build step. On platforms where the git build already worked
there is no functional difference.

Fixes #2871

## Type of Change

- [x] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [ ] Performance improvement
- [ ] Code refactoring (no functional changes)

## Changes Made

- `headroom/mcp_registry/install.py` (`build_serena_spec`): `--from
git+https://github.com/oraios/serena` -> `--from serena-agent`.
- `headroom/cli/wrap.py` (Serena `project index` pre-warm): same swap.
- `tests/test_mcp_registry/test_install.py`: updated the spec assertion
and added `test_build_serena_spec_uses_pypi_not_git_source` (asserts
`serena-agent` is used and no `git+` source remains).
- `tests/test_cli/test_wrap_serena_boost.py`: the pre-index test now
asserts `serena-agent` is in the command and the git source is not.

## Testing

- [x] Unit tests pass (`pytest`)
- [x] Linting passes (`ruff check .`)
- [x] Type checking passes (`mypy headroom`)
- [x] New tests added for new functionality
- [ ] Manual testing performed

### Test Output

```text
# Fail-before (source swap stashed, updated tests kept):
tests/test_mcp_registry/test_install.py::test_build_serena_spec_uses_agent_context FAILED
tests/test_mcp_registry/test_install.py::test_build_serena_spec_uses_pypi_not_git_source FAILED
tests/test_cli/test_wrap_serena_boost.py::test_preindex_runs_serena_in_cwd FAILED

# Pass-after:
tests/test_mcp_registry/ tests/test_cli/test_wrap_serena_boost.py
tests/test_cli/test_serena_migrate.py tests/test_cli/test_serena_disable.py   135 passed

# uvx ruff@0.15.17 check  -> All checks passed!
# uvx mypy@1.20.2 headroom/mcp_registry/install.py -> Success: no issues found in 1 source file
```

## Real Behavior Proof

- Environment: Windows 11, Python 3.12.11, project venv, pytest 9.1.1,
ruff 0.15.17 and mypy 1.20.2 via uvx.
- Exact command / steps: confirmed `serena-agent` exists on PyPI
(v1.6.1, homepage github.com/oraios/serena) and that its
`pyproject.toml` declares `[project.scripts] serena =
"serena.cli:top_level"`, so the `serena start-mcp-server ...` invocation
is unchanged. Swapped both `--from` sources, then fail-before with `git
stash push headroom/mcp_registry/install.py headroom/cli/wrap.py` (the
two production-asserting tests fail on the old git source) and
pass-after with `git stash pop` (135 serena-suite tests pass). Verified
no `git+https://github.com/oraios/serena` references remain in
`headroom/`.
- Observed result: `build_serena_spec` and the pre-index command now
install Serena from the `serena-agent` PyPI wheel, so a proot
environment gets the prebuilt wheel instead of a from-source build that
cannot hardlink. The migration/ledger tests, which use the old git spec
as a deliberately-stale fixture, are unaffected.
- Not tested: a live `headroom wrap codex` on a real proot/Termux device
(not available here). The change is a package-source swap verified
against Serena's own published package metadata and the existing
spec/command tests.

## Review Readiness

- [x] I have performed a self-review
- [x] This PR is ready for human review

## Checklist

- [x] My code follows the project's style guidelines
- [x] I have performed a self-review of my code
- [x] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [x] I did **not** edit `CHANGELOG.md`: it is generated by
release-please from my Conventional Commit PR title (a CI guard enforces
this)

## Additional Notes

The git source was unpinned (tracked the repo default branch), so
switching to `serena-agent` from PyPI does not lose a version pin; if
anything it is more reproducible. The issue reporter also noted that
`headroom wrap codex` force-rewrites the Serena block in
`~/.codex/config.toml` from this template on every launch, which is why
the fix has to live in the package source rather than a user config edit
-- this PR puts it there.

headroom/cli/wrap.py

@@ -1710,8 +1710,10 @@ def _index_serena_project(*, verbose: bool = False) -> None:
         result = run(
             [
                 "uvx",
+                # PyPI (prebuilt wheels), not the git source that fails to build
+                # under proot-based filesystems (#2871).
                 "--from",
-                "git+https://github.com/oraios/serena",
+                "serena-agent",
                 "serena",
                 "project",
                 "index",

headroom/mcp_registry/install.py

@@ -59,8 +59,11 @@ def build_serena_spec(context: str) -> ServerSpec:
         name="serena",
         command="uvx",
         args=(
+            # The PyPI package (serena-agent) ships prebuilt wheels; the git
+            # source forced a from-source build that fails under proot-based
+            # filesystems where uv cannot hardlink into a build venv (#2871).
             "--from",
-            "git+https://github.com/oraios/serena",
+            "serena-agent",
             "serena",
             "start-mcp-server",
             "--project-from-cwd",

tests/test_cli/test_wrap_serena_boost.py

@@ -122,7 +122,9 @@ def test_preindex_runs_serena_in_cwd(tmp_path: Path, monkeypatch: pytest.MonkeyP
     cmd = args[0]
     assert cmd[0] == "uvx"
     assert cmd[-3:] == ["serena", "project", "index"]
-    assert "git+https://github.com/oraios/serena" in cmd
+    # PyPI package with prebuilt wheels, not the git source (#2871).
+    assert "serena-agent" in cmd
+    assert "git+https://github.com/oraios/serena" not in cmd
     assert kwargs["cwd"] == str(tmp_path)  # invoked in the project cwd
     assert "timeout" in kwargs  # timeout-guarded
 

tests/test_mcp_registry/test_install.py

@@ -89,8 +89,9 @@ def test_build_serena_spec_uses_agent_context() -> None:
     assert spec.name == "serena"
     assert spec.command == "uvx"
     assert spec.args == (
+        # PyPI package with prebuilt wheels, not the git source (#2871).
         "--from",
-        "git+https://github.com/oraios/serena",
+        "serena-agent",
         "serena",
         "start-mcp-server",
         "--project-from-cwd",
@@ -102,6 +103,15 @@ def test_build_serena_spec_uses_agent_context() -> None:
     assert spec.env == {}
 
 
+def test_build_serena_spec_uses_pypi_not_git_source() -> None:
+    """Serena is installed from the PyPI package (prebuilt wheels), not the git
+    source, which forces a from-source build that fails under proot-based
+    filesystems where uv cannot hardlink into a build venv (#2871)."""
+    spec = build_serena_spec("codex")
+    assert "serena-agent" in spec.args
+    assert not any("git+" in arg for arg in spec.args)
+
+
 def test_build_serena_spec_disables_dashboard_popup_by_default() -> None:
     # Headroom installs Serena by default; the dashboard browser tab must not
     # auto-open. The flag overrides the user's serena_config.yml at startup,