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/db.js

@@ -575,6 +575,15 @@ try {
   db.prepare("ALTER TABLE dashboard_runs ADD COLUMN tmux_session TEXT").run();
 }
 
+// Migrate: add `worktree` to dashboard_runs — 1 when the run's pane works in a
+// git worktree of its own (`claude --worktree=<name>`, or a resume of such a
+// session), so the session view points its git panel there.
+try {
+  db.prepare("SELECT worktree FROM dashboard_runs LIMIT 1").get();
+} catch {
+  db.prepare("ALTER TABLE dashboard_runs ADD COLUMN worktree INTEGER NOT NULL DEFAULT 0").run();
+}
+
 // Migrate: add stage-detection columns to lanes. Inference is never evidence —
 // these are additive columns separate from `stage` (the declared stage), so
 // `stage`'s meaning is untouched and turning the feature off loses nothing.