feat(workspace): a session in its own worktree, Claude's way

Desktop-parity phase 6, following Claude Desktop:
- the workspace home's Worktree checkbox starts the pane with
  `claude --worktree=<name>`, so Claude Code makes, populates
  (.worktreeinclude, WorktreeCreate hook) and cleans up
  <repo>/.claude/worktrees/<name>; AgentHub only excludes
  /.claude/worktrees/ at the repo root (Claude does not)
- server/lib/session-worktree.js resolves a session's worktree from the
  session row only (owned by the workspace, registered by git worktree
  list); every lane git route takes ?session= and answers ENOWORKTREE
  instead of falling back to the main folder
- the session view's Changes panel works in the opened run's worktree
  (dashboard_runs.worktree, GitTarget in the client api)
- Resume/Fork start in the session's worktree; a gone one is ENOWORKTREE
- Archive of a worktree session removes the worktree after asking:
  git worktree remove without --force, git branch -d (unmerged work keeps
  its branch), a lock held by a live pid refused, a stale one cleared
- listWorktrees reads `locked <reason>` locks
- tests scrub GIT_* env so the pre-commit suite cannot act on this repo

server/lib/git-exclude.js

@@ -58,4 +58,25 @@ function excludeFromGit(laneDir, relPaths) {
   }
 }
 
-module.exports = { excludeFromGit };
+/**
+ * The same, anchored at the REPOSITORY root rather than `dir` — for paths a
+ * tool creates at the root whatever subdirectory it was started in (Claude
+ * Code's `.claude/worktrees/`). Best effort, like `excludeFromGit`.
+ */
+function excludeFromRepoRoot(dir, relPaths) {
+  try {
+    const env = { ...process.env };
+    for (const name of REPO_VARS) delete env[name];
+    const root = execFileSync("git", ["rev-parse", "--show-toplevel"], {
+      cwd: dir,
+      env,
+      encoding: "utf8",
+      stdio: ["ignore", "pipe", "ignore"],
+    }).trim();
+    if (root) excludeFromGit(root, relPaths);
+  } catch {
+    /* not a repo */
+  }
+}
+
+module.exports = { excludeFromGit, excludeFromRepoRoot };