[M2] Complete the issue claim lifecycle

[igbopharaoh]

Tracking

• Milestone: M2 - Real GitHub App Beta
• Suggested labels: priority:p1, type:feature, area:api, area:github, milestone:github-beta

Context

Complete issue claiming beyond the initial /claim command.

Scope

• Add /release for the current claimant.
• Define maintainer override behavior or explicitly defer it.
• Mark a claim completed when a merged PR references the claimed issue.
• Release or invalidate claims when issues close, where appropriate.
• Scope dashboard claim views to active and recent claims.

Acceptance criteria

• A claimant can release their own claim.
• A merged PR by the claimant can complete a claim.
• Claim status transitions are tested and visible in the dashboard.

Validation

• Test command authorization and merge/close transitions.

Definition of done

• Relevant Rust, API, or web tests pass.
• Documentation and runbook notes are updated when behavior changes.
• The pull request includes a concise manual verification note.