fix(repair): preflight unwritable automerge branches

[Takhoffman]

Summary

• add a shared source branch writability policy for repair execution
• preflight repair_contributor_branch source PRs before target checkout, validation planning, and Codex write preflight
• convert unwritable fork branches with maintainer_can_modify=false into explicit replacement-PR repair mode

Validation

• pnpm run build
• pnpm run build:repair
• node --test test/repair/execute-fix-policy.test.ts
• node --test test/repair/execute-fix-artifact-source.test.ts
• pnpm run check