Document agent-browser interactive test workflow in AGENTS.md

We needed a reliable, repeatable checklist for manual browser validation while
a page is already open. Recent testing showed that loading `/` alone can leave
the UI non-interactive unless a pane-target route is selected.

Architecture decisions:
- Keep this guidance in repo-level `AGENTS.md` so all contributors and agents
  use the same interactive verification flow.
- Document the exact auto-connect + pane-discovery sequence instead of generic
  browser instructions to reduce false negatives during manual checks.
- Include a concrete assertion command (`AGENT_BROWSER_OK`) to verify terminal
  input/output, not just navigation.

What was implemented:
- Added a new "Interactive testing with `agent-browser`" section.
- Documented the quick flow for attaching to an already-running browser,
  switching from `/` to `/api/state.html`, and selecting a pane route.
- Added explicit behavior notes about `/t/%NN` pane URLs being required for
  terminal interaction.
- Added a minimal interaction assertion that validates terminal IO.

Current state:
Interactive testing instructions now cover the known routing pitfall and provide
an executable sanity-check sequence for agent-browser sessions.

Next steps for colleague:
1. Use this flow whenever validating live UI behavior in an already-open browser.
2. Keep the command examples updated if agent-browser CLI flags change.
3. If pane routing behavior changes, revise the "Important behavior" notes so
   they continue to match runtime behavior.

Author: dhamidi

AGENTS.md

@@ -3,3 +3,46 @@
 We have no users.
 
 Make changes fearlessly instead of deprecating features.
+
+## E2E testing
+
+Use the Playwright test to validate the browser-to-tmux flow headlessly.
+
+Prerequisites:
+- `tmux` installed and available in `PATH`
+- Node.js/npm available
+- Go toolchain available
+
+Commands:
+- `go test ./...`
+- `npm ci`
+- `npx playwright install chromium`
+- `npm run test:e2e`
+
+Notes:
+- The E2E harness creates and tears down its own tmux session via `scripts/setup-e2e-tmux-fixture.sh`.
+- Keep the test headless and deterministic; do not depend on manual browser interaction.
+
+## Interactive testing with `agent-browser`
+
+Use `agent-browser` when you need to manually validate the running web UI in an already-open browser session.
+
+Quick flow:
+- Attach to the running browser: `agent-browser --auto-connect tab list`
+- If the app is on `/`, switch to pane discovery: `agent-browser --auto-connect open http://localhost:8080/api/state.html`
+- Snapshot interactive controls and click a pane link (for example `@e1`):
+  - `agent-browser --auto-connect snapshot -i`
+  - `agent-browser --auto-connect click @e1`
+- Confirm you are on a pane route (`/t/%NN`) and terminal input exists:
+  - `agent-browser --auto-connect get url`
+  - `agent-browser --auto-connect snapshot -i`
+
+Important behavior:
+- `/` alone is usually not interactive for terminal input because the UI needs a pane target from the URL.
+- Pane routes look like `/t/%13` and are required for terminal interaction.
+
+Minimal interaction assertion:
+- `agent-browser --auto-connect fill @e1 "echo AGENT_BROWSER_OK"`
+- `agent-browser --auto-connect press Enter`
+- `agent-browser --auto-connect get text ".xterm-rows"`
+- Verify `AGENT_BROWSER_OK` appears in the terminal output.