fix(sync-gbrain): Windows compat — capability check + brain-sync skip

[jakehann11]

Summary

Two Windows-only failures surfaced running /sync-gbrain on Git Bash from a freshly-set-up local-stdio gbrain install. Both trace to the same family of issues — Node/Bun spawnSync can't directly execute bash scripts on Windows (ENOENT). Same shape as #1571 (different code path).

1. Capability check fails on Windows. The ## Step 4: Refresh ## GBrain Search Guidance block capability check uses:

echo "ping" | gbrain put "$SLUG" >/dev/null 2>&1

Windows Git Bash treats this as writing to /dev/stdin, which gbrain rejects with ENOENT: no such file or directory, open '/dev/stdin'. CAPABILITY_OK always reports 0 on Windows even when the engine is healthy and search works — so the skill removes the ## GBrain Search Guidance block from CLAUDE.md on every run, defeating the whole point of the workflow.

Switching to gbrain put "$SLUG" --content "ping" works on both Windows and Unix, and matches the v1.42.0.0 wave (#1571 era) that rewrote all 10 user-facing put_page instruction templates to the put + --content canonical shape.

2. runBrainSyncPush produces spurious ERR rows when artifacts_sync_mode=off. The orchestrator didn't check the user's opt-out before invoking gstack-brain-sync (also a bash script). On Windows the spawn fails with result.status === undefined, producing this verdict row even with a healthy install + intentional opt-out:

ERR    brain-sync   gstack-brain-sync exited undefined (0.0s)

New readArtifactsSyncMode() parses ~/.gstack/config.yaml directly via readFileSync — no process spawn, fully portable across all platforms. When the key reads off (the default), the stage returns ran=false ok=true summary="skipped (artifacts_sync_mode=off — user opt-out)".

Verified on Windows Git Bash

Before:

gstack-gbrain-sync (incremental):
  OK    code         synced gstack-code-… (page_count=386) (7.4s)
  OK    memory       gbrain import: 3 imported, 0 unchanged, 0 failed (2.9s)
  ERR   brain-sync   gstack-brain-sync exited undefined (0.0s)

  2 ok, 1 error, 0 skipped

After:

gstack-gbrain-sync (incremental):
  OK    code         synced gstack-code-… (page_count=386) (7.0s)
  OK    memory       gbrain import: 1 imported, 0 unchanged, 0 failed (2.7s)
  SKIP  brain-sync   skipped (artifacts_sync_mode=off — user opt-out) (0.0s)

  3 ok, 0 error, 0 skipped

Capability check: CAPABILITY_OK=1 (was 0).

Test plan

• Manual verification on Windows Git Bash (alumni-watch repo, local-stdio gbrain v0.35.8.0, pglite engine)
• bun test test/gstack-gbrain-sync.test.ts — same 31 pass / 6 fail before and after this PR. The 6 failures (sourceLocalPath describe block) are pre-existing Windows-specific test issues — the tests use bash-script shims that Windows Node spawnSync can't execute via ENOENT. Same root cause family, separate fix needed.
• Unix sanity check — would appreciate a maintainer running /sync-gbrain once on macOS/Linux to confirm both paths still behave (no regression expected; the YAML-parse helper is just a readFileSync + regex and the --content flag has been the canonical shape since v0.18+).

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