fix(deepagents-vfs): surface fatal init failures from grep/glob/ls (#426)

## Summary
- `_wait_ready()` only blocked on the ready event. A failed
index-provisioning step or a failed initial sync was already recorded in
`init_errors`, but `grep`/`glob`/`ls` proceeded anyway and returned
empty results indistinguishable from "nothing matched" — the package's
worst failure mode: confidently wrong empty results.
- Fatal init failures (index provisioning or initial sync raising
outright) are now tracked separately from a watcher-start failure, which
doesn't invalidate already-synced data. `_wait_ready()` raises
`AdapterError` (`E1004_INDEX_PROVISION_FAILED` /
`E1005_INITIAL_SYNC_FAILED`) in the fatal case, which the
`adapter_boundary` decorator turns into a proper error DTO for callers.

## Test plan
- [x] Added `TestSearchSurfacesInitFailures`: `grep` errors when index
provisioning failed, `glob` errors when initial sync failed, `ls` still
succeeds when only the watcher failed to start (watched these fail
before the fix).
- [x] `uv run pytest tests/unit_tests/test_backend.py -v` — all pass.
- [x] `just lint` / `just typing` — clean.

Signed-off-by: Casey Clements <casey.clements@mongodb.com>