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/opencode/opencode.jsonc

@@ -2,7 +2,7 @@
 	"$schema": "https://opencode.ai/config.json",
 	"autoupdate": true,
 	"default_agent": "lead",
-	"plugin": ["@tarquinen/opencode-dcp"],
+	"plugin": ["@tarquinen/opencode-dcp", "./plugins/tmux-panes.ts"],
 	"agent": {
 		"general": {
 			"disable": true