feat: @agor/stores package (3 stores) + 58 BackendAdapter tests

@agor/stores:
- theme.svelte.ts, notifications.svelte.ts, health.svelte.ts extracted
- Original files replaced with re-exports (zero consumer changes needed)
- pnpm workspace + Vite/tsconfig aliases configured

BackendAdapter tests (58 new):
- backend-adapter.test.ts: 9 tests (lifecycle, singleton, testing seam)
- tauri-adapter.test.ts: 28 tests (invoke mapping, command names, params)
- electrobun-adapter.test.ts: 21 tests (RPC names, capabilities, stubs)

Total: 523 tests passing (was 465, +58)

ui-electrobun/src/bun/btmsg-db.ts

@@ -334,6 +334,42 @@ export class BtmsgDb {
     return id;
   }
 
+  // ── Feature 7: Channel membership management ─────────────────────────────
+
+  joinChannel(channelId: string, agentId: string): void {
+    // Validate channel exists
+    const ch = this.db.query<{ id: string }, [string]>(
+      "SELECT id FROM channels WHERE id = ?"
+    ).get(channelId);
+    if (!ch) throw new Error(`Channel '${channelId}' not found`);
+
+    this.db.query(
+      "INSERT OR IGNORE INTO channel_members (channel_id, agent_id) VALUES (?1, ?2)"
+    ).run(channelId, agentId);
+  }
+
+  leaveChannel(channelId: string, agentId: string): void {
+    this.db.query(
+      "DELETE FROM channel_members WHERE channel_id = ? AND agent_id = ?"
+    ).run(channelId, agentId);
+  }
+
+  getChannelMembers(channelId: string): Array<{ agentId: string; name: string; role: string }> {
+    return this.db.query<{
+      agent_id: string; name: string; role: string;
+    }, [string]>(
+      `SELECT cm.agent_id, a.name, a.role
+       FROM channel_members cm
+       JOIN agents a ON cm.agent_id = a.id
+       WHERE cm.channel_id = ?
+       ORDER BY a.name`
+    ).all(channelId).map(r => ({
+      agentId: r.agent_id,
+      name: r.name,
+      role: r.role,
+    }));
+  }
+
   // ── Heartbeats ───────────────────────────────────────────────────────────
 
   heartbeat(agentId: string): void {