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>

.agents/skills/gstack-browse/SKILL.md

@@ -457,6 +457,7 @@ Refs are invalidated on navigation — run `snapshot` again after `goto`.
 | Command | Description |
 |---------|-------------|
 | `chain` | Run commands from JSON stdin. Format: [["cmd","arg1",...],...] |
+| `inbox [--clear]` | List messages from sidebar scout inbox |
 
 ### Tabs
 | Command | Description |

.agents/skills/gstack/SKILL.md

@@ -585,6 +585,7 @@ Refs are invalidated on navigation — run `snapshot` again after `goto`.
 | Command | Description |
 |---------|-------------|
 | `chain` | Run commands from JSON stdin. Format: [["cmd","arg1",...],...] |
+| `inbox [--clear]` | List messages from sidebar scout inbox |
 
 ### Tabs
 | Command | Description |

SKILL.md

@@ -591,6 +591,7 @@ Refs are invalidated on navigation — run `snapshot` again after `goto`.
 | Command | Description |
 |---------|-------------|
 | `chain` | Run commands from JSON stdin. Format: [["cmd","arg1",...],...] |
+| `inbox [--clear]` | List messages from sidebar scout inbox |
 
 ### Tabs
 | Command | Description |

browse/SKILL.md

@@ -463,6 +463,7 @@ Refs are invalidated on navigation — run `snapshot` again after `goto`.
 | Command | Description |
 |---------|-------------|
 | `chain` | Run commands from JSON stdin. Format: [["cmd","arg1",...],...] |
+| `inbox [--clear]` | List messages from sidebar scout inbox |
 
 ### Tabs
 | Command | Description |

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

browse/src/meta-commands.ts

@@ -327,6 +327,66 @@ export async function handleMetaCommand(
       }
     }
 
+    // ─── Inbox ──────────────────────────────────────────
+    case 'inbox': {
+      const { execSync } = await import('child_process');
+      let gitRoot: string;
+      try {
+        gitRoot = execSync('git rev-parse --show-toplevel', { encoding: 'utf-8', stdio: ['pipe', 'pipe', 'pipe'] }).trim();
+      } catch {
+        return 'Not in a git repository — cannot locate inbox.';
+      }
+
+      const inboxDir = path.join(gitRoot, '.context', 'sidebar-inbox');
+      if (!fs.existsSync(inboxDir)) return 'Inbox empty.';
+
+      const files = fs.readdirSync(inboxDir)
+        .filter(f => f.endsWith('.json') && !f.startsWith('.'))
+        .sort()
+        .reverse(); // newest first
+
+      if (files.length === 0) return 'Inbox empty.';
+
+      const messages: { timestamp: string; url: string; userMessage: string }[] = [];
+      for (const file of files) {
+        try {
+          const data = JSON.parse(fs.readFileSync(path.join(inboxDir, file), 'utf-8'));
+          messages.push({
+            timestamp: data.timestamp || '',
+            url: data.page?.url || 'unknown',
+            userMessage: data.userMessage || '',
+          });
+        } catch {
+          // Skip malformed files
+        }
+      }
+
+      if (messages.length === 0) return 'Inbox empty.';
+
+      const lines: string[] = [];
+      lines.push(`SIDEBAR INBOX (${messages.length} message${messages.length === 1 ? '' : 's'})`);
+      lines.push('────────────────────────────────');
+
+      for (const msg of messages) {
+        const ts = msg.timestamp ? `[${msg.timestamp}]` : '[unknown]';
+        lines.push(`${ts} ${msg.url}`);
+        lines.push(`  "${msg.userMessage}"`);
+        lines.push('');
+      }
+
+      lines.push('────────────────────────────────');
+
+      // Handle --clear flag
+      if (args.includes('--clear')) {
+        for (const file of files) {
+          try { fs.unlinkSync(path.join(inboxDir, file)); } catch {}
+        }
+        lines.push(`Cleared ${files.length} message${files.length === 1 ? '' : 's'}.`);
+      }
+
+      return lines.join('\n');
+    }
+
     default:
       throw new Error(`Unknown meta command: ${command}`);
   }

browse/src/sidebar-agent.ts

@@ -23,6 +23,46 @@ let lastLine = 0;
 let authToken: string | null = null;
 let isProcessing = false;
 
+// ─── File drop relay ──────────────────────────────────────────
+
+function getGitRoot(): string | null {
+  try {
+    const { execSync } = require('child_process');
+    return execSync('git rev-parse --show-toplevel', { encoding: 'utf-8', stdio: ['pipe', 'pipe', 'pipe'] }).trim();
+  } catch {
+    return null;
+  }
+}
+
+function writeToInbox(message: string, pageUrl?: string, sessionId?: string): void {
+  const gitRoot = getGitRoot();
+  if (!gitRoot) {
+    console.error('[sidebar-agent] Cannot write to inbox — not in a git repo');
+    return;
+  }
+
+  const inboxDir = path.join(gitRoot, '.context', 'sidebar-inbox');
+  fs.mkdirSync(inboxDir, { recursive: true });
+
+  const now = new Date();
+  const timestamp = now.toISOString().replace(/:/g, '-');
+  const filename = `${timestamp}-observation.json`;
+  const tmpFile = path.join(inboxDir, `.${filename}.tmp`);
+  const finalFile = path.join(inboxDir, filename);
+
+  const inboxMessage = {
+    type: 'observation',
+    timestamp: now.toISOString(),
+    page: { url: pageUrl || 'unknown', title: '' },
+    userMessage: message,
+    sidebarSessionId: sessionId || 'unknown',
+  };
+
+  fs.writeFileSync(tmpFile, JSON.stringify(inboxMessage, null, 2));
+  fs.renameSync(tmpFile, finalFile);
+  console.log(`[sidebar-agent] Wrote inbox message: ${filename}`);
+}
+
 // ─── Auth ────────────────────────────────────────────────────────
 
 async function refreshToken(): Promise<string | null> {
@@ -203,6 +243,8 @@ async function poll() {
     if (!entry.message && !entry.prompt) continue;
 
     console.log(`[sidebar-agent] Processing: "${entry.message}"`);
+    // Write to inbox so workspace agent can pick it up
+    writeToInbox(entry.message || entry.prompt, entry.pageUrl, entry.sessionId);
     try {
       await askClaude(entry);
     } catch (err) {