perf(base): trim agent-browser footprint ~960MB→~625MB before release

Size concern ahead of the v1.6.0 base rebuild. agent-browser drives the full
chrome (verified headless: open/title/eval with the headless_shell removed), so
Playwright's chromium_headless_shell-* build is dead weight — drop it (~334MB),
and clean apt/npm caches in-layer. Net browser footprint ~625MB/arch.

CI disk is otherwise fine: build-base's 'Reclaim runner disk' step frees
~20-30GB (hostedtoolcache/dotnet/android/jvm) + docker prune before buildx.
hadolint clean.

CHANGELOG.md

@@ -32,8 +32,9 @@ Pre-v1.0.0 tags followed the pi npm version (`v{pi_version}[letter]`).
   `--with-deps` dependency resolution verified (the t64 renames are handled).
   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>`.
+  pointer so agents discover the capability. Adds ~625 MB (Chromium; Playwright's
+  unused headless-shell build is dropped and the apt/npm caches cleaned in-layer
+  to stay lean). 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,