feat(matrix): deep ergonomic integration — pipe, react, rooms, wormhole-pipe, ctx

turtle-matrix-bridge:
  - `send` reads stdin when piped (cmd | mx now works)
  - `pipe` — stdin → formatted code block, --lang flag for syntax hint
  - `reply <event_id> <text>` — threaded Matrix reply
  - `react <event_id> <emoji>` — emoji reaction on any event
  - `edit <event_id> <new_text>` — in-place message edit
  - `rooms` — list joined rooms with display names
  - `messages [n]` — fetch last n messages with sender + timestamp
  - `wormhole-pipe` — stdin → temp file → wormhole send → posts code to room
  - `ctx` — post active.json + mesh tail as a context snapshot
  - `-r <room>` flag on all commands (no positional-only room arg)
  - Smart lang detection from file extension + content shebang/prefix
  - `_lang_for(path)` maps 15 extensions to highlight languages

Shell surface:
  - `mx` — reads stdin when piped; `k3s get pods | mx` just works
  - `mxpipe [--lang <l>]` — pipe stdin as code block
  - `mxw` — detects pipe mode: `k3s logs pod | mxw` sends via wormhole
  - `mxreply <eid> <text>` — threaded reply
  - `mxreact <eid> <emoji>` — reaction
  - `mxedit <eid> <text>` — edit message
  - `mxrooms` — list joined rooms
  - `mxlog [n]` — print last n messages from room
  - `mxctx` — post context snapshot to room

Palette: 9 Matrix entries; all new verbs reachable via CMD+P

Author: mdheller