test: sidebar CSS interaction E2E — HN comment highlight round-trip

New E2E test (periodic tier, ~$2/run) that exercises the full sidebar
agent pipeline with CSS interaction:
1. Agent navigates to Hacker News
2. Clicks into the top story's comments
3. Reads comments and identifies the most insightful one
4. Highlights it with a 4px solid orange outline via style injection

Tests: navigation, snapshot, text reading, LLM judgment, CSS modification.
Requires real browser + real Claude (ANTHROPIC_API_KEY).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

test/helpers/touchfiles.ts

@@ -152,6 +152,7 @@ export const E2E_TOUCHFILES: Record<string, string[]> = {
   // Sidebar agent
   'sidebar-navigate':              ['browse/src/server.ts', 'browse/src/sidebar-agent.ts', 'browse/src/sidebar-utils.ts', 'extension/**'],
   'sidebar-url-accuracy':          ['browse/src/server.ts', 'browse/src/sidebar-agent.ts', 'browse/src/sidebar-utils.ts', 'extension/background.js'],
+  'sidebar-css-interaction':       ['browse/src/server.ts', 'browse/src/sidebar-agent.ts', 'browse/src/write-commands.ts', 'browse/src/read-commands.ts', 'browse/src/cdp-inspector.ts', 'extension/**'],
 
   // Autoplan
   'autoplan-core':  ['autoplan/**', 'plan-ceo-review/**', 'plan-eng-review/**', 'plan-design-review/**'],
@@ -282,6 +283,7 @@ export const E2E_TIERS: Record<string, 'gate' | 'periodic'> = {
   // Sidebar agent
   'sidebar-navigate': 'periodic',
   'sidebar-url-accuracy': 'periodic',
+  'sidebar-css-interaction': 'periodic',
 
   // Autoplan — periodic (not yet implemented)
   'autoplan-core': 'periodic',