feat: add tmux integration with visual subagent panes

- plugins/tmux-panes.ts: opencode plugin that hooks into session.created/
  session.deleted events to spawn a tmux pane per subagent running
  'opencode attach', giving live visual TUI for each background agent
- opencode.jsonc: load the local plugin alongside @tarquinen/opencode-dcp
- skills/tmux-session/SKILL.md: teach agents to manage persistent tmux
  sessions (dev servers, watchers, worktree windows) with oc- naming
- c.fish / cc.fish: auto-start a tmux session when invoked outside tmux
  so the visual panes plugin can always activate

.config/fish/functions/c.fish

@@ -1,3 +1,7 @@
-function c --wraps=opencode --description 'alias c opencode'
-    opencode $argv
+function c --wraps=opencode --description 'opencode (auto-starts tmux for visual subagent panes)'
+    if not set -q TMUX
+        tmux new-session opencode $argv
+    else
+        opencode $argv
+    end
 end