fix(brain-sync): fail loud when artifacts source has zero pages

[jzeisweiss]

Summary

• stop reporting a successful artifact git command as gbrain indexing
• verify the maintained artifact source page count with a read-only, five-second bounded probe
• fail loud with index_failed when that source reports zero pages; otherwise report the git result and explicitly mark index freshness as unverified
• terminate the full probe process tree on timeout, including Git Bash/MSYS native cleanup

This deliberately does not run gbrain sync at every skill boundary: that operation is expensive and owns a single-writer database. Missing, unavailable, invalid, or nonzero probe results remain truthful indexing not verified outcomes rather than false indexing claims.

Refs jzeisweiss/my_dog_molly#3047.

Verification

Red first on upstream main: the zero-page regression returned status 0 before this change. The final test proves a real artifact commit reaches the remote, then the command fails because the maintained source still has zero pages.

• bun test test/brain-sync.test.ts test/gstack-gbrain-sync.test.ts test/brain-sync-windows-paths.test.ts test/gbrain-spawn-windows-shell.test.ts — 122 pass, 0 fail
• bash -n bin/gstack-brain-sync
• shellcheck --severity=error bin/gstack-brain-sync
• git diff --check

The broader free suite completed every shard except one unrelated timeout-wrapper timing assertion under six-way load (rc=143 vs rc=124); that assertion passes standalone on both this commit and pristine upstream main.

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[jzeisweiss]

Rebased onto current main (ad840054, v1.69.0.0). Same commit, new parent.

Local verification after rebase: bun test test/brain-sync.test.ts test/gstack-gbrain-sync.test.ts → 97 pass, 0 fail.