feat(launch): label the throwaway --launch profile + document escape hatches (issue #9)

A bare --launch opens an isolated empty profile (no cookies/login/
extensions). A human watching the desktop sees a mystery Chrome window
under an unfamiliar profile and reads it as broken/suspicious.

- Seed the temp profile's Local State (profile.info_cache.Default.name,
  the field Chrome's profile chip reads) + Default/Preferences with
  'agent-browser (<session>)', so the window self-identifies which agent
  session owns it.
- Rewrite the --launch warning to explain it's an isolated test profile and
  point at the escape hatches: --profile auto / AGENT_BROWSER_PROFILE=auto
  to reuse real Chrome, and --args "--load-extension=<dir>" for extensions.
- SKILL.md documents the same.

Adds a unit test for the profile-label writer.

skill-data/core/SKILL.md

@@ -73,6 +73,17 @@ popup never fires. `agent-browser extension connect` is the explicit form of the
 same path. Zero-confirmation, zero-token. Use `--launch` instead when a fresh,
 isolated browser is fine.
 
+`--launch` opens an **isolated, empty test profile** — no cookies, no login, no
+extensions (so the extension-relay path is off). Its window is labelled
+`agent-browser (<session>)` in Chrome's profile menu so a human watching the
+desktop knows which session owns it. If a launched session needs more:
+
+- **Real cookies / login / extensions** → drop `--launch`, use `--profile auto`
+  (reuses the user's real Chrome profile), or set `AGENT_BROWSER_PROFILE=auto`
+  once so every call does it by default.
+- **A specific unpacked extension in the test profile** →
+  `--launch --args "--load-extension=<dir>"`.
+
 **If you DO hit the "Allow remote debugging?" dialog**, don't keep retrying (every
 attempt re-pops it). One of two things is true: