feat(identity): cache official miner detection with outage-safe behavior

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Parent: #32

Goal

Make confirmed-miner gating reliable and cheaper without ever guessing publicly.

Requirements

• Cache official Gittensor miner detection with a short TTL.
• Store status as confirmed, not_found, or unavailable.
• On official API outage, fail closed for public GitHub output.
• Add audit events for cache hits, misses, unavailable source, and public-surface skips.
• Do not use GitHub history alone to publicly classify someone as a miner.

Acceptance Criteria

• Confirmed miner PRs can use cached status within TTL.
• Non-miners stay quiet.
• API unavailable produces no public comment, label, or check.
• Maintainer diagnostics explain detection unavailable without exposing private data.

Testing Requirements

This issue must preserve the repo's CI gate:

• npm run test:ci must pass.
• Global coverage must remain at or above 95% for lines, statements, functions, and branches.
• Add or update tests for every new branch, fallback path, sanitizer rule, and regression.
• Add invariant/property-style tests when behavior depends on sorting, gating, public/private boundaries, scoring, queue pressure, or source-upload safety.
• Public GitHub output must be tested against forbidden language: wallet, hotkey, raw trust score, payout, reward estimate, farming, private reviewability, and public score estimate.

Required Test Scenarios

• Cache hit confirmed.
• Cache hit not_found.
• Cache expiry.
• API unavailable fail-closed.
• GitHub history does not override official source.
• Public sanitizer regression.