Request: merge targetBranch run/reap support

[EugeneTrapeznikov]

Request

Please review targetBranch run support from this fork branch.

Fork PR: EugeneTrapeznikov#1
Upstream compare: main...EugeneTrapeznikov:warren:feat/target-branch-runs
Commit: f9761db

Why

Some workflows need follow-up runs that start from an existing branch and have Warren reap push back to that same branch. Today POST /runs accepts ref, but reap still pushes the unique burrow branch, so repair/review/verify loops create a new branch instead of updating the branch they were dispatched from.

What changed

• Adds nullable runs.target_branch to SQLite/Postgres schemas and generated migrations.
• Wires targetBranch through POST /runs, spawn input, repo persistence, client types, and UI API types.
• Defaults spawn refresh ref to targetBranch when ref is omitted.
• Forwards the resolved base ref to Burrow as baseBranch so the unique workspace branch starts from the target branch tip.
• Keeps the sandbox on the unique burrow run branch.
• Reap pushes HEAD: when targetBranch is set, preserving the historical HEAD: behavior otherwise.
• Adds tests for spawn refresh/persistence, HTTP dispatch, client mapping, Burrow baseBranch forwarding, and reap push target.

Validation

bun run check:all passed locally.

[EugeneTrapeznikov]

Additional validation: tested this with smoke workflows that dispatch follow-up runs against an existing PR branch. The run starts on the unique workspace branch, reaps with targetBranch set, and the existing PR is consistently updated instead of creating a new PR branch.