fix(wrap): keep agent savings opt-in (#1294)

## Description

Fixes a regression from #830 where `headroom wrap codex` / `claude` /
`cursor` treated `agent-90` as required even when the user started a
normal proxy without `HEADROOM_SAVINGS_PROFILE`.

A plain `headroom proxy` on port 8787 followed by `headroom wrap codex`
currently reports `Proxy on port 8787 is missing: --savings-profile` and
tries to restart the already-running proxy. The `agent-90` profile was
documented as opt-in, so wrap should only require or inject it when
`HEADROOM_SAVINGS_PROFILE` is explicitly set.

Closes #1293

## 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

- Stop defaulting agent wrappers to `agent-90` when
`HEADROOM_SAVINGS_PROFILE` is unset.
- Stop reporting agent-savings config mismatches unless an agent savings
profile was explicitly requested.
- Add regression tests for default wrap startup, explicit profile
forwarding, and reuse/restart behavior around existing proxies.
- Fix repo-wide pre-commit issues found during amend: Windows-safe
`fcntl` typing, an optional env typing issue, OpenCode JSON parser
return typing, and ruff import/format drift.

## Testing

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

### Test Output

```text
> maturin develop -m crates/headroom-py/Cargo.toml
Finished `dev` profile [unoptimized + debuginfo] target(s) in 27.41s
Built wheel for abi3 Python >= 3.10
Installed headroom-ai-0.27.0

> C:\git\headroom\.venv\Scripts\python.exe -c "from headroom._core import hello; print(hello())"
headroom-core

> ruff check .
All checks passed!

> ruff format --check .
913 files already formatted

> C:\git\headroom\.venv\Scripts\python.exe -m mypy headroom
Success: no issues found in 388 source files

> C:\git\headroom\.venv\Scripts\python.exe -m pytest tests/test_agent_savings.py tests/test_cli/test_wrap_persistent.py tests/test_proxy_healthchecks.py tests/test_transforms/test_content_router.py -q
collected 112 items
112 passed, 1 warning in 16.04s

> git diff --check
# no output

> git commit --amend --no-edit
Sync plugin versions.....................................................Passed
ruff.....................................................................Passed
ruff-format..............................................................Passed
mypy.....................................................................Passed
```

## Real Behavior Proof

- Environment: Windows dev checkout, Python 3.13.3 via
`C:\git\headroom\.venv\Scripts\python.exe`, Rust extension built with
`maturin develop -m crates/headroom-py/Cargo.toml`.
- Exact command / steps: reproduced the code path from #830 by
exercising `_ensure_proxy(8787, False, agent_type="codex")` with a
running proxy health payload that has no `savings_profile`, and by
exercising `_start_proxy(8787, agent_type="codex")` with
`HEADROOM_SAVINGS_PROFILE` unset and set.
- Observed result: without `HEADROOM_SAVINGS_PROFILE`, wrap reuses the
running proxy and `_start_proxy` does not inject
`HEADROOM_SAVINGS_PROFILE` or `HEADROOM_TARGET_RATIO`; with
`HEADROOM_SAVINGS_PROFILE=agent-90`, wrap still requires/forwards the
profile and restarts an incompatible proxy.
- Not tested: full end-to-end CLI launch against a live Codex binary.
The focused proxy/wrap tests cover the failing restart/config decision
directly.

## 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
- [ ] I have updated the CHANGELOG.md if applicable

## Screenshots (if applicable)

N/A

## Additional Notes

The pytest run emits an existing Windows `cp1252` background-thread
warning while reading subprocess output; the tests still pass.

No documentation or changelog update is included because this restores
the already-documented opt-in behavior for `agent-90`.

tests/test_cli/test_wrap_codex.py

@@ -761,10 +761,10 @@ def test_start_proxy_uses_separate_session_for_signal_isolation(
 
 
 @pytest.mark.parametrize("agent_type", ["claude", "codex", "cursor"])
-def test_start_proxy_applies_agent_90_defaults(
+def test_start_proxy_does_not_apply_agent_90_defaults(
     monkeypatch: pytest.MonkeyPatch, tmp_path: Path, agent_type: str
 ) -> None:
-    """Wrapped coding agents should start the proxy with high-savings defaults."""
+    """Wrapped coding agents keep agent-savings opt-in by default."""
     popen_kwargs: dict[str, object] = {}
 
     class FakeProc:
@@ -785,10 +785,10 @@ def test_start_proxy_applies_agent_90_defaults(
 
     env = popen_kwargs["env"]
     assert isinstance(env, dict)
-    assert env["HEADROOM_SAVINGS_PROFILE"] == "agent-90"
-    assert env["HEADROOM_TARGET_RATIO"] == "0.10"
-    assert env["HEADROOM_MAX_ITEMS"] == "8"
-    assert env["HEADROOM_SMART_CRUSHER_COMPACTION"] == "0"
+    assert "HEADROOM_SAVINGS_PROFILE" not in env
+    assert "HEADROOM_TARGET_RATIO" not in env
+    assert "HEADROOM_MAX_ITEMS" not in env
+    assert "HEADROOM_SMART_CRUSHER_COMPACTION" not in env
 
 
 def test_start_proxy_preserves_explicit_savings_overrides(