fix(orchestrator): capture sidecar stderr + raise default boot timeout (closes #1)

[evannadeau]

Closes #1.

Three changes against mcp/server.ts, plus the rebuilt dist/server.js:

1. Sidecar stdout/stderr → <pluginRoot>/.sidecar.log (truncated each boot). trySpawn accepts a log FD; replaces stdio: "ignore". FD opened once in startSidecar, shared across all uvx → python → python3 fallback attempts. Falls back to "ignore" if openSync throws — preserves prior behavior on open failure.
2. ORCH_SIDECAR_BOOT_TIMEOUT_MS env override + uvx default 60 s → 180 s. Eliminates the residential-link failure described in orchestrator: silent sidecar boot failure on first install_embeddings — 60s timeout exceeded by cold-start downloads #1. Python/python3 fallback timeouts honor the env var but default unchanged at 30 s (assumes uvx already cached deps).
3. system_status and install_embeddings now reference the log path and, when the bge-m3 model isn't yet cached, suggest the timeout env var. install_embeddings(check) adds a bge-m3 model cache: present (~10s boot expected) | not yet downloaded (~2 GB on first boot) line that disambiguates first-run from broken-run.

.gitignore picks up .sidecar.log. dist/server.js rebuilt via bun run build.

Tested

• bun run typecheck — clean.
• bun test — 330 pass / 1 fail. The single failure is in tests/hooks/hooks.test.ts (session-activity-nudge text mismatch) and reproduces against upstream/main without this patch — pre-existing, not introduced here.
• Cold-start verified end-to-end on residential broadband (~100 Mbps WSL2). Reconstructed timing from process lstart + HF cache blob mtimes:
  • 10:00:41 — cold sidecar boot started (HF cache empty)
  • 10:01:54 — 2.27 GB model.onnx_data finished downloading (73 s in)
  • ~10:02:02 — Model ready, port file written, sidecar healthy (~80 s total)
  • The prior 60 s budget would have killed the spawn at 10:01:41, mid-download.
• .sidecar.log captured the previously-discarded boot diagnostics (download progress, HF rate-limit warning, ONNX load, port write, "Listening on …").
• install_embeddings(check) with cache populated returned the expected bge-m3 model cache: present (~10s boot expected) line.

Known minor limitation

.sidecar.log opens in "w" mode per startSidecar call, so a second startSidecar (e.g. on install_embeddings(install) after an automatic session-start spawn) truncates the prior attempt's log. Strictly better than stdio: "ignore", but consider switching to "a" (append) or rotate-on-open if maintainers want multi-attempt forensics. Happy to follow up.

Not changed

• No version bump — leaving that to maintainer judgment.
• No new tests — adding integration tests for spawn/log behavior would be heavy. Existing suite + manual cold-start cover the change.
• No README/docs changes — the env var surfaces in install_embeddings(install) failure output, which is where users hit it.