Task: Implement the fleet command

Add a compass fleet subcommand that lists every git worktree with its branch, mapped story + task (via the task #+branch: field), and open PR (live state via gh), so parallel checkouts/agents don't collide.

• compass fleet lists every worktree from git worktree list with branch, mapped story + task, and open PR + live state.
• The branch → task mapping is read from each worktree's own tree (the task #+branch: field), so it works on unmerged branches.
• The current worktree is marked; worktrees with no matching task (e.g. main) are shown plainly.
• -f json emits a structured array; works offline (omits PR state).

1. list_worktrees (parse git worktree list --porcelain).
2. task_for_branch (per-worktree glob of task_*.org for #+branch:).
3. open_prs_by_branch (one gh pr list call, matched by headRefName).
4. cmd_fleet + fleet subparser; pretty + json output.

Dogfooded across the five live worktrees: local1 mapped to its story, task and PR #828; PRs matched live by branch; the current worktree flagged with →.

Implemented compass fleet in compass.py (cmd_fleet plus list_worktrees / task_for_branch / open_prs_by_branch helpers and the fleet subparser). Verified across the five live worktrees: local1 → "ORE Studio Emacs dashboard" / its audit task / PR #828; this worktree self-maps to this very task; the current worktree is marked →; -f json emits the structured array. where / add unaffected; py_compile clean.