Add POST /tasks/:taskId/branches/link to link existing branches without GitHub create #16

Problem

POST /projects/:projectId/tasks/:taskId/branches always calls GitHub to create the branch before inserting into github_task_branches.

When developers create branches in their IDE or agent and only want to associate an existing branch with a task, the API returns:

GitHub API error: Reference already exists

Pull request linking already supports this pattern via POST .../pull-requests/link. Branches had no equivalent.

Proposed solution

Add a link-only endpoint:

POST /projects/:projectId/tasks/:taskId/branches/link

Body:

{
  "repo_id": "uuid",
  "branch_name": "feature/my-branch"
}

Expected behavior:

1. Verify repo_id belongs to the project
2. Verify the branch ref exists on GitHub
3. Insert into github_task_branches (ON CONFLICT DO NOTHING)
4. Emit github.branch_linked
5. Allow one branch → many tasks

Also add MCP tool github_link_branch_to_task, mirroring github_link_pr_to_task.

Status

Implemented in a single clean commit — see open PR from branch feat/link-branch-clean (supersedes earlier draft PR #17).

Use case

Branch and PR are created outside Paca (IDE, agent, GitHub web). Tasks are linked afterward for traceability — same workflow as pull-requests/link.