feat(github-app): add command response previews

[JSONbored]

Parent roadmap: #82

Background

Maintainers should know exactly what Gittensory would post before enabling visible command responses.

Goal

Add preview endpoints/tools for sanitized @gittensory command responses without posting comments.

Current Behavior

Mention commands can respond, but previewing public output before enabling or invoking commands is limited.

Desired Behavior

Maintainers can preview help/preflight/blockers/duplicate-check/miner-context/next-action output for sample PRs.

Implementation Requirements

• Add preview API for installed repo maintainers/admins.
• Render command-specific public-safe responses.
• Show skip/comment/label/check decision.
• Include missing permission diagnostics.
• Never post during preview.

Public/Private Output Boundaries

Public GitHub output must stay quiet-by-default and sanitized. It must not include private reviewability, scoreability, reward/risk, raw trust, wallet/hotkey, or public score estimates.

Acceptance Criteria

• Preview does not create comments, checks, or labels.
• Confirmed miner preview differs from non-miner skip.
• Missing permissions are explained.
• Output passes sanitizer tests.

Testing Requirements

• npm run test:ci must pass.
• Global coverage must remain at or above 97% for lines, statements, functions, and branches.
• Aim for 98%+ branch coverage locally to avoid CI variance.
• Add tests for every new branch, fallback path, sanitizer rule, and regression.
• Add invariant/property-style tests when behavior depends on sorting, gating, scoring, queue pressure, source-upload safety, public/private boundaries, or upstream drift.
• Public GitHub output must be tested against forbidden language: wallet, hotkey, raw trust score, payout, reward estimate, farming, private reviewability, and public score estimate.
• MCP/local tooling must prove source contents are not uploaded.

Additional Test Scenarios

• Confirmed miner preview.
• Non-miner preview.
• Maintainer author preview.
• Missing Issues write fixture.
• Forbidden-language regression.