feat: implement /kill and auto-clean dead topic state

[DefaultPerson]

Summary

/kill was advertised as a BotCommand menu entry but had no handler — the
catch-all MessageHandler(filters.COMMAND, ...) forwarded the literal text
/kill into the Claude Code TUI. This PR adds the handler and, along the way,
fixes the orphan-state buildup that affects every path where a topic dies
(close, delete, /kill, dead window).

Background

When a window goes away, three state files can retain stale entries:

• state.json — window_states, window_display_names, user_window_offsets
• session_map.json — Claude Code's SessionStart hook writes it but emits no
  "session ended" event
• monitor_state.json

The 60s probe in status_polling only matched Topic_id_invalid. In private
chats Telegram returns ok=true from unpinAllForumTopicMessages regardless of
message_thread_id, so DM-mode topic deletions are never detected via the probe
and the worker logs Failed to send message: Message thread not found once per
second indefinitely.

Changes

• bot.kill_command — kills the tmux window, drops the binding, purges
  window state, removes the session_map.json entry, and calls
  delete_forum_topic. Registered before the catch-all MessageHandler.
• session.SessionManager — adds purge_window,
  remove_session_map_entry, and cleanup_dead_topic. Reused by
  kill_command, topic_closed_handler, and the status_polling cleanup
  paths.
• status_polling — probe matches both Topic_id_invalid and
  Message thread not found; interval lowered from 60s to 5s.
• message_sender.send_with_fallback / safe_send — on a final
  BadRequest containing either marker, call
  session_manager.cleanup_dead_topic(chat_id, thread_id). This is the only
  path that detects DM-mode topic deletion.
• scripts/restart.sh — sweeps any .venv/bin/ccbot processes outside the
  target tmux pane. Multiple instances polling the same bot token produce
  Conflict: terminated by other getUpdates request and silently break command
  routing.
• Style — one-line ruff format fix in bot.py for a pre-existing
  violation that broke ruff format --check on main.

Test plan

• uv run ruff check src/ tests/ and ruff format --check src/ tests/
• uv run pyright src/ccbot/ — no new errors (two pre-existing
  resolved_chat F821 errors at lines 1144/1522 are untouched)
• uv run pytest -q — 240 passed (test_kill_command.py adds 5 cases);
  one pre-existing unrelated failure on test_format_tool_result_text[Write]
• Manual: /kill in a bound topic kills the window, deletes the topic,
  purges all three state files
• Manual: deleting a topic via Telegram UI triggers cleanup within ~1s
  via the send-error path
• Manual: restart.sh kills a stray ccbot instance running in another
  tmux server