feat(profile): --profile auto + stop steering users into temp-profile launches

Addresses the footgun raised in issue #1 follow-up: plain `--launch` silently
uses a temporary EMPTY profile (no cookies/login), and the connect-failure
error even recommended it — trapping agents into thinking they reused the
logged-in browser when they didn't.

- `--profile auto`: resolves to the Chrome profile last used (from Local State
  `profile.last_used`), falling back to "Default", then the first profile. So
  `--launch --profile auto open <url>` reuses real login state without naming
  the profile. (--profile <name>/Default already worked.)
- connect-failure error now recommends `--launch --profile auto` and states
  plainly that bare `--launch` is a temporary EMPTY profile — no cookies/login.
- bare `--launch` (no --profile, not CI) now prints a warning to that effect.
- README: fix Setup (relaunch with --remote-debugging-port, not chrome://inspect)
  and split Standalone mode into throwaway vs. keep-your-login (`--profile auto`).

Tests: resolve_chrome_profile("auto") prefers last_used, falls back to Default.

README.md

@@ -93,14 +93,24 @@ agent-browser screenshot ./page.png
 
 The agent operates in your Chrome — you'll see tabs opening, pages loading, clicks happening in real time. You can take over at any point (e.g. solve a CAPTCHA), then let the agent continue.
 
-### Standalone mode
+### Standalone mode (`--launch`)
 
-If you need a separate browser (CI, testing, etc.):
+Spawn a separate browser instead of attaching to your running Chrome:
 
 ```bash
+# Throwaway: fresh, EMPTY profile — no cookies, no login (good for CI/testing)
 agent-browser --launch open https://example.com
+
+# Keep your login: launch with your real Chrome profile (cookies/sessions intact)
+agent-browser --launch --profile auto open https://x.com/home
+# or name it explicitly: --profile Default / --profile "Profile 1"
 ```
 
+> ⚠️ Plain `--launch` (no `--profile`) uses a **temporary empty profile** — you will
+> NOT be logged into anything. For logged-in sites use `--profile auto` (picks the
+> Chrome profile you used most recently) or `--profile <name>`. agent-browser prints
+> a warning when you `--launch` without a profile.
+
 In CI environments, standalone mode is used automatically.
 
 ## Anti-detection