feat(tabs): tab-to-tab agent communication via short handles

Add send_to_tab / read_tab tools so an agent can message or read another
tab by a git-style short handle (shortest unique prefix of the tab UUID,
min 4 chars), shown in the tab bar.

- core/db/tabs: resolveTabPrefix + shortestUniquePrefix (open tabs only,
  LIKE-sanitized prefix matching)
- new tools read-tab.ts / send-to-tab.ts (+ tests) decoupled from the DB
  TabRow via a minimal ResolvedTabRef projection
- agent-manager: unified deliverMessage routing (busy -> queue, idle ->
  new turn) shared by POST /chat and send_to_tab; agent->agent auto-wake
  budget (MAX_AGENT_AUTO_WAKES) to bound ping-pong loops
- summon/loader: send_to_tab + read_tab as grantable tools
- frontend: shortHandleFor + handle badge in TabBar; perm toggles
- notes: tab-comm / user-agents / todo-redesign plans
- chore: biome format fixes (debug-logger, summon.test)

Refs notes/plan-tab-comm.md

4 files changed, 40 insertions, 0 deletions

diff --git a/packages/frontend/src/lib/components/TabBar.svelte b/packages/frontend/src/lib/components/TabBar.svelte
index feb1e5b..3cbd849 100644
--- a/packages/frontend/src/lib/components/TabBar.svelte
+++ b/packages/frontend/src/lib/components/TabBar.svelte
@@ -56,6 +56,7 @@ const activeUserTabId = $derived(
 			>
 				<span class="flex items-center gap-1.5">
 					<span class="w-1.5 h-1.5 rounded-full shrink-0 {statusColor(tab.agentStatus)}"></span>
+					<span class="font-mono text-[10px] px-1 py-0.5 rounded bg-base-300 text-base-content/60 shrink-0" title="Tab ID — agents address this tab by this handle">{tabStore.shortHandleFor(tab.id)}</span>
 					<span class="max-w-32 truncate text-xs">{tab.title}</span>
 				</span>
 				<button
@@ -89,6 +90,7 @@ const activeUserTabId = $derived(
 				>
 					<span class="flex items-center gap-1">
 						<span class="w-1 h-1 rounded-full shrink-0 {statusColor(tab.agentStatus)}"></span>
+						<span class="font-mono text-[10px] px-1 rounded bg-base-300 text-base-content/60 shrink-0" title="Tab ID — agents address this tab by this handle">{tabStore.shortHandleFor(tab.id)}</span>
 						<span class="max-w-28 truncate text-xs">{tab.title}</span>
 					</span>
 					{#if tab.persistent}
diff --git a/packages/frontend/src/lib/components/ToolPermissions.svelte b/packages/frontend/src/lib/components/ToolPermissions.svelte
index d64eaf9..0caf0fa 100644
--- a/packages/frontend/src/lib/components/ToolPermissions.svelte
+++ b/packages/frontend/src/lib/components/ToolPermissions.svelte
@@ -28,6 +28,16 @@ const toolPermissions: ToolPermission[] = [
 		description: "Allow the AI to open new independent top-level tabs",
 	},
 	{
+		id: "send_to_tab",
+		label: "Message other tabs",
+		description: "Allow the AI to send messages to other tabs by their ID",
+	},
+	{
+		id: "read_tab",
+		label: "Read other tabs",
+		description: "Allow the AI to read other tabs' latest responses by their ID",
+	},
+	{
 		id: "web_search",
 		label: "Web search",
 		description: "Allow the AI to search the web via Firecrawl",
diff --git a/packages/frontend/src/lib/settings.svelte.ts b/packages/frontend/src/lib/settings.svelte.ts
index 11eb79c..123008d 100644
--- a/packages/frontend/src/lib/settings.svelte.ts
+++ b/packages/frontend/src/lib/settings.svelte.ts
@@ -9,6 +9,8 @@ let toolPerms = $state<Record<string, boolean>>({
 	bash: false,
 	summon: false,
 	user_agent: false,
+	send_to_tab: false,
+	read_tab: false,
 	external_directory: false,
 	web_search: false,
 	youtube_transcribe: false,
@@ -19,6 +21,8 @@ let savedToolPerms = $state<Record<string, boolean>>({
 	bash: false,
 	summon: false,
 	user_agent: false,
+	send_to_tab: false,
+	read_tab: false,
 	external_directory: false,
 	web_search: false,
 	youtube_transcribe: false,
diff --git a/packages/frontend/src/lib/tabs.svelte.ts b/packages/frontend/src/lib/tabs.svelte.ts
index e303c80..317de8d 100644
--- a/packages/frontend/src/lib/tabs.svelte.ts
+++ b/packages/frontend/src/lib/tabs.svelte.ts
@@ -232,6 +232,29 @@ export function createTabStore() {
 		return tabs.find((t) => t.id === id);
 	}
 
+	/**
+	 * Minimum display length of a tab handle (git-style short id). Mirrors
+	 * `MIN_TAB_PREFIX_LENGTH` in core's `db/tabs.ts` so the handle the user sees
+	 * is always resolvable by the backend's `resolveTabPrefix`.
+	 */
+	const MIN_HANDLE_LENGTH = 4;
+
+	/**
+	 * Compute the shortest unique prefix (≥ MIN_HANDLE_LENGTH chars) of `tabId`
+	 * among all currently-open tabs — the displayed "handle" agents use to
+	 * address each other. Purely DERIVED from the UUIDs already in `tabs`; never
+	 * stored. Grows by one char only when another open tab shares the prefix, and
+	 * shrinks back when that sibling closes.
+	 */
+	function shortHandleFor(tabId: string): string {
+		const others = tabs.map((t) => t.id).filter((id) => id !== tabId);
+		for (let len = MIN_HANDLE_LENGTH; len < tabId.length; len++) {
+			const candidate = tabId.slice(0, len);
+			if (!others.some((id) => id.startsWith(candidate))) return candidate;
+		}
+		return tabId;
+	}
+
 	async function createNewTab(): Promise<Tab> {
 		const id = generateId();
 		const title = "New Tab";
@@ -1926,6 +1949,7 @@ export function createTabStore() {
 		get configReloaded() {
 			return configReloaded;
 		},
+		shortHandleFor,
 		createNewTab,
 		switchTab,
 		closeTab,