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/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
+```