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'
+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

.config/fish/functions/cc.fish

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

.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

.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

.config/opencode/skills/tmux-session/SKILL.md

@@ -0,0 +1,100 @@
+---
+name: tmux-session
+description: Manage persistent terminal sessions in tmux for long-running processes, dev servers, and interactive tools — load when a task needs a background process or interactive shell
+permalink: opencode-config/skills/tmux-session/skill
+---
+
+## When to Use This Skill
+
+Load this skill when a task requires:
+- Running a **dev server or watcher** that must stay alive across multiple tool calls (e.g. `npm run dev`, `cargo watch`, `pytest --watch`)
+- An **interactive REPL or debugger** that needs to persist state between commands
+- Running a process **in the background** while working in the main pane
+- **Parallel worktree work** where each feature branch gets its own named window
+
+Do NOT use tmux for one-shot commands that complete and exit — use `bash` directly for those.
+
+## Naming Convention
+
+All opencode-managed sessions and windows use the `oc-` prefix:
+
+| Resource | Name pattern | Example |
+|---|---|---|
+| Named session | `oc-<project>` | `oc-myapp` |
+| Named window | `oc-<feature>` | `oc-auth-refactor` |
+| Background process window | `oc-bg-<process>` | `oc-bg-dev-server` |
+
+## Starting a Persistent Session
+
+```bash
+# Check if already inside tmux
+echo $TMUX
+
+# Start a new named session (detached) for a long-running process
+tmux new-session -d -s oc-bg-dev-server "npm run dev"
+
+# Or in a new window within the current session
+tmux new-window -n oc-bg-dev-server "npm run dev"
+```
+
+## Sending Commands to a Running Session
+
+```bash
+# Send a command to a named session
+tmux send-keys -t oc-bg-dev-server "npm run build" Enter
+
+# Read the last N lines of output from a pane
+tmux capture-pane -t oc-bg-dev-server -p | tail -20
+```
+
+## Checking if a Session/Window Exists
+
+```bash
+# Check for a named session
+tmux has-session -t oc-bg-dev-server 2>/dev/null && echo "running" || echo "not running"
+
+# List all oc- prefixed windows in current session
+tmux list-windows -F "#{window_name}" | grep "^oc-"
+```
+
+## Worktree + Window Workflow
+
+When working across multiple git worktrees, open each in its own tmux window:
+
+```bash
+# Create worktree and open it in a dedicated window
+git worktree add .worktrees/auth-refactor -b auth-refactor
+tmux new-window -n oc-auth-refactor -c .worktrees/auth-refactor
+```
+
+Switch between worktrees by switching windows:
+```bash
+tmux select-window -t oc-auth-refactor
+```
+
+## Cleanup
+
+Always clean up sessions and windows when done:
+
+```bash
+# Kill a specific window
+tmux kill-window -t oc-bg-dev-server
+
+# Kill a detached session
+tmux kill-session -t oc-bg-dev-server
+
+# Kill all oc- prefixed windows in current session
+tmux list-windows -F "#{window_name}" | grep "^oc-" | xargs -I{} tmux kill-window -t {}
+```
+
+## Checking Process Output
+
+Before assuming a background process is healthy, capture its recent output:
+
+```bash
+# Capture last 30 lines of a pane
+tmux capture-pane -t oc-bg-dev-server -p -S -30
+
+# Check if process is still running (exit code 0 = alive)
+tmux has-session -t oc-bg-dev-server 2>/dev/null
+```