Skip baseline walk on git workspaces

`prepareRun` used to call `captureWorkspaceBaseline` on every
workspace with a path, even when `.git` was present. The baseline was
silently discarded by `collectChangedPaths` (the git branch wins) but
the walk still paid for itself — an O(tree) scan on every run start,
capped at 20k files, with the only effect being that the first 20k
entries got captured before the capper truncated the tail.

Collapse the decision into `captureBaselineIfRequired`: the function
short-circuits to null for git workspaces so `git status` stays the
sole source of truth, and only scans the tree when the baseline is
the only source available. The call site in `runtime-bridge` now
passes through without the per-call conditional.

src/main/agent-runtime/artifact-projector.ts

@@ -110,17 +110,24 @@ const BASELINE_SKIP_DIR_NAMES: ReadonlySet<string> = new Set([
 ]);
 
 /**
- * Walk `workspacePath` and record the mtime of every non-directory
- * entry. Directories matching `BASELINE_SKIP_DIR_NAMES` are pruned at
- * walk time — the same categories the registrability filter rejects —
- * so the baseline stays bounded on real project trees. Stops early at
+ * Capture a run-start mtime baseline only for workspaces that need
+ * one. Git workspaces return null here because `git status` already
+ * provides the authoritative diff — paying for an O(tree) walk on
+ * every run just to throw the result away is a hot-path tax the
+ * scheduler should not be paying.
+ *
+ * For non-git workspaces, walk the workspace and record the mtime of
+ * every non-directory entry. Directories matching
+ * `BASELINE_SKIP_DIR_NAMES` are pruned at walk time — the same
+ * categories the registrability filter rejects — so the baseline
+ * stays bounded on real project trees. The walker stops early at
  * `MAX_BASELINE_ENTRIES` and flags `truncated`; the partial baseline
- * still lets the diff detect every file it saw, while the operator is
- * told via the skip reason that an oversize workspace fell off the
- * tail.
+ * still covers every path that was captured while the operator sees
+ * the cap signal.
  */
-export function captureWorkspaceBaseline(workspacePath: string): WorkspaceBaseline | null {
+export function captureBaselineIfRequired(workspacePath: string | null): WorkspaceBaseline | null {
   if (!workspacePath) return null;
+  if (isGitRepo(workspacePath)) return null;
   try {
     if (!statSync(workspacePath).isDirectory()) return null;
   } catch {