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

client/src/i18n/locales/en/lanes.json

@@ -351,6 +351,10 @@
   "tree.splitHint": "Live pane — Ctrl/Cmd+click opens it in a second pane",
   "tree.renameSession": "Rename session",
   "tree.archiveSession": "Archive — hide from this workspace (deletes nothing)",
+  "tree.worktreeSession": "In its own worktree: .claude/worktrees/{{name}}",
+  "tree.archiveWorktreeTitle": "Archive and remove its worktree?",
+  "tree.archiveWorktreeBody": "Like Claude Desktop, archiving removes the session's worktree .claude/worktrees/{{name}} and its branch. Git refuses if it holds uncommitted changes, and a branch with commits merged nowhere is kept — nothing unsaved is lost.",
+  "tree.archiveWorktreeConfirm": "Archive and remove",
   "tree.archived_one": "{{count}} archived session",
   "tree.archived_other": "{{count}} archived sessions",
   "tree.unarchive": "Unarchive",
@@ -361,6 +365,8 @@
   "home.worktreeCount_other": "{{count}} worktrees",
   "home.placeholder": "How can I help you today?",
   "home.start": "Start",
+  "home.worktree": "Worktree",
+  "home.worktreeHint": "Run this session in its own git worktree — <repo>/.claude/worktrees/, a branch of its own, made and cleaned up by Claude Code",
   "home.allBusy": "All four panes of this workspace are busy — stop one before starting another session.",
   "home.recents": "Recents",
   "home.noRecents": "No sessions yet — start one above.",