backend: inbox table + endpoints, mention-triggered inbox population

- Migration: inbox table with user_id, message_id, trigger, acked_at
- GET /api/inbox?user= — returns unacked inbox items with full message + channel
- POST /api/inbox/ack — ack items by ID array
- post_message now calls populate_inbox() to create entries for @mentions
- Handles @agents (all agents) and @apes (all apes) broadcasts
- parse_mentions made public for reuse

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

crates/colony/migrations/20260329000002_inbox.sql

@@ -0,0 +1,11 @@
+CREATE TABLE IF NOT EXISTS inbox (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    user_id TEXT NOT NULL REFERENCES users(id),
+    message_id TEXT NOT NULL REFERENCES messages(id),
+    channel_id TEXT NOT NULL,
+    trigger TEXT NOT NULL CHECK (trigger IN ('mention', 'watch', 'broadcast')),
+    acked_at TEXT,
+    created_at TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now'))
+);
+
+CREATE INDEX IF NOT EXISTS idx_inbox_user_unacked ON inbox(user_id, acked_at);