docs: add persistent dev server guidance to paperclip-dev skill

Agents launching dev servers with pnpm dev during a heartbeat lose the
process when the heartbeat shell exits. Document using detached tmux
sessions to keep dev servers alive for manual testing, with session
management commands and verification steps. Learned from PAPA-81 where
QA repeatedly found the server dead between heartbeats.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

skills/paperclip-dev/SKILL.md

@@ -159,6 +159,56 @@ These rules exist because agents have caused real damage by improvising around C
 
 5. **Seeding is a CLI operation.** When asked to seed a worktree database from the main instance, use `worktree reseed` or recreate with `worktree:make --seed-mode full`. Read `doc/DEVELOPING.md` for the full option tables. Never attempt manual database copying.
 
+## Persistent Dev Servers (for Manual Testing)
+
+When an agent needs to start a dev server that outlives the current heartbeat — for example, so a human or QA agent can manually test against it — the server process **must** be launched in a detached session. A process started directly from a heartbeat shell is killed when the heartbeat exits.
+
+### Use `tmux` for persistent servers
+
+```bash
+# 1. cd into the worktree (or main repo) and source the environment
+cd <worktree-path>
+eval "$(npx paperclipai worktree env)"   # skip if using the primary instance
+
+# 2. Start the dev server in a named, detached tmux session
+tmux new-session -d -s <session-name> 'pnpm dev'
+
+# Example with a descriptive name:
+tmux new-session -d -s papa81-auth-3102 'pnpm dev'
+```
+
+### Managing the session
+
+| Task | Command |
+|------|---------|
+| Check if the session is alive | `tmux has-session -t <session-name> 2>/dev/null && echo running` |
+| View server output | `tmux capture-pane -t <session-name> -p` |
+| Kill the session | `tmux kill-session -t <session-name>` |
+| List all tmux sessions | `tmux list-sessions` |
+
+### Verifying the server is reachable
+
+After launching, confirm the port is listening before reporting success:
+
+```bash
+# Wait briefly for startup, then verify
+sleep 3
+curl -sf http://127.0.0.1:<port>/api/health && echo "Server is up"
+lsof -nP -iTCP:<port> -sTCP:LISTEN
+```
+
+### Why this matters
+
+In PAPA-81, the dev server was started with `pnpm dev` directly from the agent shell. It appeared healthy during the heartbeat, but died as soon as the heartbeat exited. QA repeatedly found the port unreachable seconds later. Switching to a detached `tmux` session solved the problem — the server survived across heartbeats and remained available for manual testing.
+
+### Key rules
+
+1. **Always use `tmux` (or equivalent)** when a dev server needs to stay running after the heartbeat ends.
+2. **Name the session descriptively** — include the issue or worktree name and port (e.g., `papa81-auth-3102`).
+3. **Verify the server is listening** before reporting the URL to anyone.
+4. **Do not use `nohup` or `&` alone** — these are unreliable for agent shells that may have their entire process group killed.
+5. **Clean up when done** — kill the tmux session when the testing is complete.
+
 ## Common Mistakes
 
 | Mistake | Fix |
@@ -172,3 +222,4 @@ These rules exist because agents have caused real damage by improvising around C
 | Running agents against wrong instance | Verify `PAPERCLIP_API_URL` points to the correct port |
 | CLI command fails | Do NOT work around it — report the error and block (see Hard Rules above) |
 | Agent tries manual postgres operations | NEVER do this — all DB ops go through the CLI (see Hard Rules above) |
+| Dev server dies between heartbeats | Launch in a detached `tmux` session — see "Persistent Dev Servers" above |