refactor: drop unused shared.embedder global (#149)

The module-level `shared.embedder` global was written by `create_embedder()`
but never read from production code — the embedder flows through the
`EMBEDDER` ContextKey via `context.provide` / `use_context`.

Drop the global along with the two test readers that kept it alive:

- `tests/test_daemon.py` — the `daemon_sock` fixture pre-loaded an embedder
  and monkeypatched `dm.create_embedder` to reuse it. That was only useful
  as a cross-module cache via the now-dead global; with a session-scoped
  fixture, `run_daemon()` loads the embedder once from the saved settings
  regardless.
- `tests/test_chunker_registry.py` — removed a stale
  `monkeypatch.setattr(_shared, "embedder", stub)` whose comment claimed
  CodeChunk.embedding read the global at schema resolution time. It reads
  the EMBEDDER ContextKey instead; the stub is already wired through
  `Project.create(..., stub, ...)`.