feat: file drop relay + $B inbox command

Sidebar agent now writes structured messages to .context/sidebar-inbox/
when processing user input. The workspace agent can read these via
$B inbox to see what the user reported from the browser.

File drop format:
  .context/sidebar-inbox/{timestamp}-observation.json
  { type, timestamp, page: {url}, userMessage, sidebarSessionId }

Atomic writes (tmp + rename) prevent partial reads. $B inbox --clear
removes messages after display.

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

browse/src/commands.ts

@@ -32,6 +32,7 @@ export const META_COMMANDS = new Set([
   'url', 'snapshot',
   'handoff', 'resume',
   'connect', 'disconnect', 'focus',
+  'inbox',
 ]);
 
 export const ALL_COMMANDS = new Set([...READ_COMMANDS, ...WRITE_COMMANDS, ...META_COMMANDS]);
@@ -103,6 +104,8 @@ export const COMMAND_DESCRIPTIONS: Record<string, { category: string; descriptio
   'connect': { category: 'Server', description: 'Launch headed Chromium with Chrome extension', usage: 'connect' },
   'disconnect': { category: 'Server', description: 'Disconnect headed browser, return to headless mode' },
   'focus':   { category: 'Server', description: 'Bring headed browser window to foreground (macOS)', usage: 'focus [@ref]' },
+  // Inbox
+  'inbox':   { category: 'Meta', description: 'List messages from sidebar scout inbox', usage: 'inbox [--clear]' },
 };
 
 // Load-time validation: descriptions must cover exactly the command sets