feat(herdr): report box state as $lilbox pane metadata (#132)

[pgebheim]

Summary

Adds a $lilbox sidebar badge to the herdr plugin: a new best-effort hook command reports each pane's box state (running / stopped / none) as pane metadata via herdr pane report-metadata <pane_id> --source lilbox --token lilbox=<state>, wired to pane.created / pane.focused events. Supports --all (per-workspace sweep via herdr pane list), though the manifest only uses the single-pane form. Reuses the existing worktree_path() / box_name() / box_status() helpers; missing herdr/lilbox/pane/box all exit 0 so the hook can never break herdr's event loop.

Closes #132

Test plan

• contrib/herdr/tests/run.sh — 29 assertions incl. new hook coverage (token reported for running box, none for boxless worktree, exit 0 without context or herdr binary, --all sweeps a workspace)
• cargo fmt --check clean (no Rust touched)
• Manual: open a workspace in herdr, confirm the $lilbox token shows in the sidebar row documented in the README

Architecture

No architectural change — bash-only plugin change, one new command + two manifest events, no new dependencies.