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/worktree.js

@@ -192,7 +192,7 @@ function slugify(text) {
  * Records are separated by blank lines, with keys like:
  * - worktree <path>
  * - branch refs/heads/<name>
- * - locked (optional, bare line)
+ * - locked [<reason>] (optional)
  */
 async function listWorktrees(sourceRepo) {
   const result = await git(sourceRepo, ["worktree", "list", "--porcelain"]);
@@ -215,8 +215,11 @@ async function listWorktrees(sourceRepo) {
       const branchPath = line.slice("branch ".length);
       // Strip refs/heads/ prefix
       current.branch = branchPath.replace(/^refs\/heads\//, "");
-    } else if (line === "locked") {
+    } else if (line === "locked" || line.startsWith("locked ")) {
+      // `locked <reason>` when one was given (Claude Code locks its session
+      // worktrees with `claude session <name> (pid …)`).
       current.locked = true;
+      if (line.length > "locked".length) current.lockReason = line.slice("locked ".length);
     }
   }