fix(mobile): hide attachments-history button on phones + regression guards

The COD-39 attachment-history header button was visible on the cramped
phone header. Hide it on phones alongside the settings gear and lifecycle
log (the mobile header is intentionally minimal — those controls live in
the toolbar). One-line addition to the existing @media (max-width: 430px)
display:none block in mobile.css.

This is the second time a header control leaked onto mobile (the
plan-usage chip was the first), so add two regression guards:

- test/mobile-header-buttons-policy.test.ts — a pure static analysis of
  index.html + mobile.css (no browser), so it runs in the normal CI sweep
  (the test/mobile/** Playwright suite is EXCLUDED from CI and never gated
  this). It enumerates every default-visible header button and fails when
  one has no phone-visibility decision — either a mobile.css hide rule or
  an explicit MOBILE_VISIBLE_ALLOWLIST entry. A new header button now
  forces that decision. Verified it fails on the pre-fix state and passes
  after.
- test/mobile/header-buttons.test.ts — real-browser E2E in the mobile
  suite: asserts the attachments/settings/lifecycle buttons are hidden on
  an emulated iPhone 14 Pro and the attachments button is visible on a
  desktop-class tablet.

tsc + lint + prettier + both new tests green. Only CSS + tests changed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/mobile/helpers/constants.ts

@@ -8,6 +8,7 @@ export const PORTS = {
   DEVICE_MATRIX: 3205,
   VISUAL_REGRESSION: 3206,
   ACCESSIBILITY: 3207,
+  HEADER_BUTTONS: 3208,
 } as const;
 
 // CSS Selectors
@@ -64,8 +65,8 @@ export const BREAKPOINTS = {
 
 // Keyboard constants (match app.js KeyboardHandler)
 export const KEYBOARD = {
-  SHOW_THRESHOLD: 150,    // heightDiff > 150px triggers show
-  HIDE_THRESHOLD: 100,    // heightDiff < 100px triggers hide
+  SHOW_THRESHOLD: 150, // heightDiff > 150px triggers show
+  HIDE_THRESHOLD: 100, // heightDiff < 100px triggers hide
   TYPICAL_IOS_HEIGHT: 336,
   FOCUSIN_DELAY: 400,
   ANIMATION_DELAY: 150,
@@ -87,7 +88,7 @@ export const SWIPE = {
 export const SUBAGENT = {
   MOBILE_CARD_HEIGHT: 110,
   MOBILE_CARD_GAP: 4,
-  MOBILE_CARD_STRIDE: 114,   // height + gap
+  MOBILE_CARD_STRIDE: 114, // height + gap
   TOOLBAR_OFFSET: 40,
   DEFAULT_HEADER_HEIGHT: 36,
 } as const;