feat(github-app): add command-specific @gittensory responses

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Problem

@gittensory mention commands exist, but several commands still produce generic top-action output. Maintainers and PR authors need command-specific, public-safe responses.

Desired behavior

Tailor public-safe output for these commands:

• @gittensory preflight
• @gittensory blockers
• @gittensory duplicate-check
• @gittensory miner-context
• @gittensory next-action

Implementation notes

• Start in src/github/commands.ts and related webhook handlers.
• Keep existing authorization: maintainers can invoke installed-repo commands; PR authors can invoke only on their own PRs when official miner detection passes.
• Non-miners, bots, unauthorized users, and miner detection outages should fail closed.
• Each command should have its own response builder instead of reusing one generic top-action block.

Public/private boundaries

Public comments must never include score estimates, payout/reward language, wallet/hotkey data, raw trust scores, or private reviewability internals. Keep comments advisory and maintainer-friendly.

Acceptance criteria

• Each supported command produces distinct output.
• Unauthorized users receive no visible public response unless an existing policy explicitly allows a sanitized denial.
• Miner context confirms only public-safe official miner status.
• Duplicate-check reports public-safe collision/WIP context without internal labels like likely_duplicate.
• Existing sticky-comment/update behavior remains non-spammy.

Tests

• Command parser coverage for every command.
• Maintainer authorization.
• PR author authorization.
• Non-miner, bot, and API-outage fail-closed cases.
• Public sanitizer regression.