Detect git workspaces with rev-parse instead of a .git stat

`isGitRepo` only accepted a `.git` directory, so linked worktrees and
submodules — both of which store `.git` as a file containing
`gitdir: …` — silently fell into the non-git branch. The baseline
diff then replaced `git status` as the source of truth for workspaces
that git itself recognises, and edge cases where the baseline is
missing (a freshly-created worktree mid-run, a submodule without a
captured snapshot) bled into `no_baseline` skips instead of running
the authoritative diff.

Call `git rev-parse --is-inside-work-tree` and take git's own yes/no
answer. Exit 0 means git would happily run `status` there; any other
outcome (no git binary, corrupted ref, genuinely non-git) lands us on
the baseline path exactly as before.

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

@@ -1,6 +1,6 @@
 import { isAbsolute, relative, resolve } from "node:path";
 import { execFileSync } from "node:child_process";
-import { existsSync, readdirSync, statSync } from "node:fs";
+import { readdirSync, statSync } from "node:fs";
 import type { AppDatabase } from "@main/database";
 
 /**
@@ -392,9 +392,28 @@ function projectBashArtifacts(
   return projectChangedPaths(db, ctx, change.paths);
 }
 
+/**
+ * Ask git whether `workspacePath` sits inside a working tree. This
+ * covers the three forms the `.git` entry takes in real setups:
+ *
+ *   - a directory (a plain clone),
+ *   - a file with `gitdir: …` (a linked worktree), or
+ *   - a file with `gitdir: …/modules/…` (a submodule).
+ *
+ * The previous `existsSync + isDirectory` probe only recognised the
+ * first form, so linked worktrees and submodules silently degraded
+ * onto the baseline path — and lost `git status` as the authoritative
+ * diff source. `rev-parse --is-inside-work-tree` exits 0 whenever git
+ * itself would accept the directory, which is the semantics we want.
+ */
 function isGitRepo(workspacePath: string): boolean {
   try {
-    return existsSync(resolve(workspacePath, ".git")) && statSync(resolve(workspacePath, ".git")).isDirectory();
+    execFileSync("git", ["rev-parse", "--is-inside-work-tree"], {
+      cwd: workspacePath,
+      timeout: 2_000,
+      stdio: ["ignore", "ignore", "ignore"],
+    });
+    return true;
   } catch {
     return false;
   }