feat: Validate default branch exists in repo when setting project defaultBranch

registerProject and updateProject now verify via remoteBranchExists that the
specified branch actually exists in the cloned repository before saving.

docs/git-process-logging.md

@@ -31,6 +31,21 @@ Worktrees stored in `.cw-worktrees/` subdirectory of the repo. Each agent gets a
 4. On conflict: `git merge --abort`, emit `worktree:conflict` with conflicting file list
 5. Restore original branch
 
+### BranchManager (`src/git/branch-manager.ts`)
+- **Port**: `BranchManager` interface
+- **Adapter**: `SimpleGitBranchManager` using simple-git
+
+| Method | Purpose |
+|--------|---------|
+| `ensureBranch(repoPath, branch, baseBranch)` | Create branch from base if it doesn't exist (idempotent) |
+| `mergeBranch(repoPath, source, target)` | Merge via ephemeral worktree, returns conflict info |
+| `diffBranches(repoPath, base, head)` | Three-dot diff between branches |
+| `deleteBranch(repoPath, branch)` | Delete local branch (no-op if missing) |
+| `branchExists(repoPath, branch)` | Check local branches |
+| `remoteBranchExists(repoPath, branch)` | Check remote tracking branches (`origin/<branch>`) |
+
+`remoteBranchExists` is used by `registerProject` and `updateProject` to validate that a project's default branch actually exists in the cloned repository before saving.
+
 ### Project Clones
 - `cloneProject(url, destPath)` — Simple git clone wrapper
 - `ensureProjectClone(project, workspaceRoot)` — Idempotent: checks if clone exists, clones if not