handle /clear in clide: reset to a fresh session instead of forwarding

Claude Code's /clear forks the conversation to a new session-id, which
clide's transcript reader (pinned to the spawn --session-id) can't
follow — so after /clear the pane froze on the old transcript and looked
dead. clide now owns /clear: it's intercepted in the composer's send
path (never forwarded to tmux), tears the pane's session down, and
respawns a fresh empty one. A new session-id is forced even for the
primary so it starts empty rather than resuming the old transcript;
_spawn's self-heal kills the stale tmux session. The old transcript is
left on disk.

Known follow-up (T-156): /resume and /compact have the same forking
problem but need different handling (a session picker, not a wipe).

T-156.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -140,6 +140,10 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
   Recognised commands are delivered as typed input so Claude's TUI parses
   them; other input (and stray leading slashes like a `/tmp` path) stays
   bracketed-pasted as literal text.
+- `/clear` is now handled by clide — it resets the Claude pane to a fresh,
+  empty session — instead of being forwarded to Claude Code, whose `/clear`
+  forked to a new session clide couldn't follow and left the pane
+  unresponsive (T-156).
 - Claude secondary panes no longer flash a false "session exited" while
   the session is alive (a transient tmux client exit is now verified
   against the live session), and the tab and banner agree on the label