fix: make the failure sentinel diagnosable (version stamp, bounded history, scoped hint)

[m3talux]

Context — diagnosed production incident (2026-06-09)

During the cross-version index incident (a stale pre-0.8 plugin cache writing v0.7 float vectors into the v0.8 int8[256] store — see the companion fence PR), the failure sentinel actively hindered diagnosis:

1. No attribution — multiple immutable plugin caches (different versions) share one ~/.superbrain; an unattributed last-failure.txt line could have been written by any of them.
2. No history — last-failure.txt is overwrite-on-write and read-and-clear, so each failure destroyed the previous one.
3. Misleading hint — sb-session-start appended "set ANTHROPIC_API_KEY if it persists" to EVERY failure, including index errors, steering diagnosis toward auth/quota.

No data was lost in the incident; this PR is purely about diagnosability next time.

Change

• src/sentinel.ts — writeFailure() stamps every failure line with the writing plugin's package version ([vX.Y.Z], resolved via pluginRoot(), cached per process; "unknown" fallback).
• Failure history: every failure is also appended to ~/.superbrain/failures.log, windowed to the last MAX_FAILURE_LOG_LINES = 100 lines (same windowed-log pattern as the reject-queue cap). The log is never read-and-cleared, so history survives the SessionStart sentinel clear. Best-effort: history can never break the sentinel write.
• bin/sb-session-start.ts — the ANTHROPIC_API_KEY hint is now scoped to failures matching /distill/i (the only failures where the API-key escape hatch helps); all other failures keep the "fixed automatically next checkpoint" copy without it.

Tests (red → green, tests/sentinelHygiene.test.ts)

• Version stamp present in last-failure.txt (was red: no stamp existed).
• History appends across multiple failures and stays bounded at exactly 100 lines, oldest rotated out (was red: no failures.log).
• Hint scoping: an index-failure message produces SessionStart output without ANTHROPIC_API_KEY; a distill-failure message keeps it (index case was red).
• Capture-failure surfacing E2E (this seam had no coverage): vec_chunks recreated as int8[8] to reproduce the incident's insert failure, then the real distill entrypoint (bin/sb-distill.ts) run with SUPERBRAIN_DISTILL_STUB — asserts the note .md IS written, exit code 0, and the sentinel contains index failed with the version stamp.

Status

• Full suite: 120 files / 796 tests passed (baseline at main: 119/790).
• dist/ rebuilt and committed; git diff --exit-code dist clean after build.
• Banked for the June 12 reassessment — DRAFT, do not merge, no release cut.

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