refactor(mt#649): Remove branchNameToTaskId and redundant self-repair in prepare-pr

[edobry]

Summary

Remove branchNameToTaskId() — it reintroduced fragile string parsing that the UUID refactor was meant to eliminate. The DB record (SessionRecord.taskId) is the source of truth for task linkage.

Also remove ~420 lines of redundant self-repair code from preparePrImpl and preparePr. The session DB is already wrapped with SessionAutoRepairProvider which handles orphaned sessions at the provider layer — the self-repair in the PR functions was dead weight.

Changes

• task-id.ts: Delete branchNameToTaskId(), update @deprecated JSDoc to reference DB lookup
• prepare-pr-operations.ts: Remove ~210-line self-repair block (TASK chore(deps-dev): bump @types/node from 22.18.3 to 24.4.0 #168), replace with simple error. Remove 7 unused imports.
• prepare-pr.ts: Same treatment — remove self-repair, simplify error
• backend-qualified-id.ts: Remove @deprecated reference to deleted function
• multi-backend-integration.test.ts: Remove 5 self-repair tests, add test verifying no self-repair at this layer

Net: -420 lines of redundant code

Test plan

• 371 tests pass in src/domain/git/
• Full test suite passes