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/plugins/tmux-panes.ts

@@ -0,0 +1,72 @@
+import type { Plugin } from "@opencode-ai/plugin"
+import { spawn } from "bun"
+
+/**
+ * tmux-panes plugin
+ *
+ * When opencode spawns a background subagent, this plugin automatically opens
+ * a new tmux pane showing that subagent's live TUI via `opencode attach`.
+ * The pane closes when the subagent session ends.
+ *
+ * Only activates when running inside a tmux session (TMUX env var is set).
+ */
+
+const isInsideTmux = () => Boolean(process.env.TMUX)
+const getCurrentPaneId = () => process.env.TMUX_PANE
+
+const plugin: Plugin = async (ctx) => {
+	if (!isInsideTmux()) return {}
+
+	const sessions = new Map<string, string>() // sessionId → tmux paneId
+	const sourcePaneId = getCurrentPaneId()
+	const serverUrl = ctx.serverUrl.toString()
+
+	return {
+		event: async ({ event }) => {
+			// Spawn a new pane when a subagent session is created
+			if (event.type === "session.created") {
+				const info = (event as any).properties?.info
+				// parentID presence distinguishes subagents from the root session
+				if (!info?.id || !info?.parentID) return
+				const sessionId: string = info.id
+				if (sessions.has(sessionId)) return
+
+				const cmd = `opencode attach ${serverUrl} --session ${sessionId}`
+				const proc = spawn(
+					[
+						"tmux",
+						"split-window",
+						"-h", // horizontal split
+						"-d", // don't focus the new pane
+						"-P",
+						"-F",
+						"#{pane_id}", // print the new pane's ID
+						...(sourcePaneId ? ["-t", sourcePaneId] : []),
+						cmd,
+					],
+					{ stdout: "pipe", stderr: "pipe" },
+				)
+
+				const paneId = (await new Response(proc.stdout).text()).trim()
+				if ((await proc.exited) === 0 && paneId) {
+					sessions.set(sessionId, paneId)
+				}
+			}
+
+			// Kill the pane when the subagent session ends
+			if (event.type === "session.deleted") {
+				const info = (event as any).properties?.info
+				const paneId = sessions.get(info?.id)
+				if (paneId) {
+					spawn(["tmux", "kill-pane", "-t", paneId], {
+						stdout: "ignore",
+						stderr: "ignore",
+					})
+					sessions.delete(info.id)
+				}
+			}
+		},
+	}
+}
+
+export default plugin