feat: add diffBranchesStat and diffFileSingle to BranchManager

Adds FileStatEntry type and two new primitives to the BranchManager
port and SimpleGitBranchManager adapter, enabling split diff
procedures in the tRPC layer without returning raw multi-megabyte diffs.

- FileStatEntry captures path, status, additions/deletions, oldPath
  (renames), and optional projectId for multi-project routing
- diffBranchesStat uses --name-status + --numstat, detects binary
  files (shown as - / - in numstat), handles spaces in filenames
- diffFileSingle returns raw unified diff for a single file path

docs/git-process-logging.md

@@ -40,6 +40,8 @@ Worktrees stored in `.cw-worktrees/` subdirectory of the repo. Each agent gets a
 | `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 |
+| `diffBranchesStat(repoPath, base, head)` | Per-file metadata (path, status, additions, deletions) — no hunk content. Binary files included with `status: 'binary'` and counts of 0. Returns `FileStatEntry[]`. |
+| `diffFileSingle(repoPath, base, head, filePath)` | Raw unified diff for a single file (three-dot diff). `filePath` must be URL-decoded. Returns empty string for binary files. |
 | `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>`) |