emacs: add pm-serve live sidebar + generic ghostel session tagging
pm-serve.el: a magit-section sidebar that streams session state from
`pm agent serve` over SSE, grouped by project, colored by status, mapping a
row back to its launching buffer via the injected PM_META_BUF. The SSE
connection is async (`:nowait`) and auto-reconnects on drop, so a daemon
restart never wedges or strands the sidebar.
pm-agent.el: inject the PM_META_* launch seeds into *every* ghostel session
via `ghostel-pre-spawn-hook` (installed by `pm-agent-setup-ghostel`), so an
agent started by hand in any ghostel buffer is tracked too — not only
`pm-agent-launch` dispatches. The ghostel dispatcher drops its now-redundant
manual env seeding.