docs(agents): point the global AGENTS.md managed block at agent-browser

Discoverability follow-up to baking agent-browser into the base: agents won't
reach for a browser they don't know they have (the exact gap that made this
capability easy to miss). Add a short pointer section to the pi-devbox managed
block — capability, the preset AGENT_BROWSER_EXECUTABLE_PATH, and
`agent-browser skills get core --full` for the command set. Pointer only; depth
stays in the skill. rootfs change → folds into the same base-<hash> rebuild.
CHANGELOG note updated.

CHANGELOG.md

@@ -30,8 +30,10 @@ Pre-v1.0.0 tags followed the pi npm version (`v{pi_version}[letter]`).
   symlink, exported as `AGENT_BROWSER_EXECUTABLE_PATH`, insulates the config
   from Playwright's per-version `chromium-<rev>` directory name. Debian trixie
   `--with-deps` dependency resolution verified (the t64 renames are handled).
-  Adds ~960 MB (Chromium + headless shell). Base-affecting, rebuilds
-  `base-<hash>`.
+  The global AGENTS.md managed block
+  (`rootfs/usr/local/share/pi-devbox/pi-global-AGENTS.append.md`) gains a short
+  pointer so agents discover the capability. Adds ~960 MB (Chromium + headless
+  shell). Base-affecting, rebuilds `base-<hash>`.
 
 - **`pi-devbox-version` command.** Wraps `/etc/pi-devbox/build-manifest.json`
   into a human-readable summary (release tag, build date, source revision,