feat(commands): @gittensory explain <finding> command — parse + classify + echo a finding's rationale (dispatch scaffold)

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Wire @gittensory explain : a contributor/maintainer asks for more detail on a specific posted review finding. The parser bounty already captures the argument; this bounty adds the classifier + authorization + a handler that looks up the referenced finding from the review-thread findings store (src/review/review-thread-findings.ts) and posts its public-safe rationale. Contributor scope = argument validation + classifier + lookup-and-echo of an ALREADY-generated finding rationale (no new AI generation — that would be a separate maintainer-owned budget concern) + tests.

Deliverables

• Add maybeProcessExplainCommand: require a non-empty finding argument (skip with a clear reason when absent), classify → evaluateCommandAuthorization("explain") → look up the finding → post its stored rationale via sanitizePublicComment
• When the finding id is unknown, post a public-safe not-found note rather than silently no-op'ing
• Do NOT call the model to generate new content in this bounty (echo the stored finding); record a github_app.finding_explained audit event
• Reuse the shared preamble/classifier + gate-override recording shape
• Unit tests: missing-argument skip, known-id echo, unknown-id not-found, authorized/unauthorized, bot/action skips (both branches of the argument-present + id-found conditionals)

References

• src/review/review-thread-findings.ts
• src/review/inline-comments.ts
• src/github/commands.ts:164 (argument capture)
• src/queue/processors.ts:5977 (handler pattern)
• #1960

Part of #1960.

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size: S · held for later release (gittensor:feature once activated)

[linear-code]

JSO-2270