feat(bounties): add bounty lifecycle watcher

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Problem

Bounty context can be active, completed, cancelled, stale, or ambiguous. Contributors need Gittensory to avoid steering them toward dead or misunderstood bounty work.

Desired behavior

Ingest bounty lifecycle state, link bounty issues to PRs where discoverable, and feed bounty status into issue quality, preflight, and MCP opportunity ranking.

Implementation notes

• Use existing bounty/lifecycle storage where possible.
• Keep active, historical, completed, cancelled, and ambiguous states distinct.
• Link solver PRs and close references when discoverable.
• Do not treat stale bounty context as active opportunity.

Public/private boundaries

Bounty advisories should be phrased as contribution context, not guaranteed compensation. Do not include wallet/hotkey or payout guarantee language.

Acceptance criteria

• Active bounty context is distinguished from completed/cancelled historical data.
• Stale bounty context warns contributors before they act on it.
• Linked PR validity is included when available.
• Issue quality and MCP opportunity ranking account for bounty state.

Tests

• Active bounty fixture.
• Completed bounty fixture.
• Cancelled bounty fixture.
• Stale bounty fixture.
• Linked PR fixture.
• Ambiguous state fixture.