fix(memory): use ONNX embedder for wrap --memory sync (#1092) (#1262)

## Description

`headroom wrap --memory` could never import memories: the startup sync
subprocess (`python -m
headroom.memory.sync`) and the in-process Codex memory import both built
their backend with
`LocalBackendConfig(db_path=...)`, which defaults `embedder_backend` to
`"local"` —
sentence-transformers + PyTorch (~2 GB). On the proxy extras that
dependency is absent, so sync
crashed with `ImportError: sentence-transformers is required for
LocalEmbedder` while the proxy
itself served memory fine via the torch-free ONNX backend. This routes
both paths through a
shared `_build_sync_backend` helper that uses `embedder_backend="onnx"`,
matching the proxy MCP
server (`headroom/memory/mcp_server.py`).

Closes #1092

## Type of Change

- [x] Bug fix (non-breaking change that fixes an issue)

## Changes Made

- `headroom/memory/sync.py`: added `_build_sync_backend(db_path)` that
constructs the backend
  with `embedder_backend="onnx"`; the sync CLI subprocess now uses it.
- `headroom/cli/wrap.py`: the in-process Claude→DB memory import (Codex
wrap path) now uses the
  same helper instead of the LOCAL-defaulting `LocalBackendConfig`.
- `tests/test_memory_sync.py`: added
`test_sync_backend_uses_onnx_embedder` regression test.

## Testing

- [x] Unit tests pass (`pytest`)
- [x] Linting passes (`ruff check .`)
- [x] New tests added for new functionality

### Test Output

```text
$ python -m pytest tests/test_memory_sync.py -q
31 passed

$ python -m ruff check headroom/memory/sync.py headroom/cli/wrap.py tests/test_memory_sync.py
All checks passed!
```

## Real Behavior Proof

- Environment: Windows 11, Python 3.13, headroom on branch
fix/1092-memory-sync-onnx-embedder
- Exact command / steps: Ran the memory-sync suite + ruff, and an import
smoke that builds the
sync backend: `python -c "from headroom.memory.sync import
_build_sync_backend;
  print(_build_sync_backend('x.db')._config.embedder_backend)"`.
- Observed result: 31 tests pass (incl. the new regression test), ruff
clean, and the smoke
prints `onnx` — the sync backend no longer defaults to the
sentence-transformers embedder.
- Not tested: Did not run a full live `headroom wrap claude --memory`
end to end (needs the
ONNX model download + Claude memory files); the same-model
(all-MiniLM-L6-v2, 384-dim) ONNX
backend the proxy already uses keeps vectors DB-compatible, so no
migration is involved.

## Review Readiness

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

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_memory_sync.py

@@ -23,6 +23,7 @@ from typing import Any
 import pytest
 
 from headroom.memory.sync import (
+    _build_sync_backend,
     sync,
     sync_export,
     sync_import,
@@ -675,3 +676,14 @@ class TestCrossAgentInterop:
         # AGENTS.md has both (from DB)
         agents_content = agents_md.read_text()
         assert "FastAPI" in agents_content or "8787" in agents_content
+
+
+def test_sync_backend_uses_onnx_embedder(tmp_path):
+    """#1092: the sync subprocess must pick the torch-free ONNX embedder.
+
+    Defaulting to the LOCAL (sentence-transformers) embedder makes
+    `wrap --memory` crash with an ImportError on the proxy extras. The backend
+    must match the proxy MCP server, which uses ONNX.
+    """
+    backend = _build_sync_backend(str(tmp_path / "memory.db"))
+    assert backend._config.embedder_backend == "onnx"