feat(compaction): add UI-driven conversation compaction

Summarize a conversation's older "head" into a structured anchored Markdown
summary while preserving the most recent turns verbatim, shrinking context size
while keeping the information needed to continue coherently. Triggered by a
"Compact conversation" button in Chat Settings (not an agent tool).

Approach informed by OpenCode's session/compaction.ts:
- Ported SUMMARY_TEMPLATE (Goal / Constraints / Progress / Key Decisions /
  Next Steps / Critical Context / Relevant Files) and the anchored-summary
  buildPrompt (re-summarizes a prior summary when present).
- Ported the TOOL_OUTPUT_MAX_CHARS (2000) cap on tool results in the summary
  request.
- Simplified tail selection to a fixed recent-turn count (DEFAULT_TAIL_TURNS=2)
  instead of OpenCode's token-budget splitTurn.

core:
- New src/compaction/ module (pure, DB-free): template, prompt builder,
  head/tail selection, transcript renderer with tool-output capping, prior
  summary extraction. Generic over ChatMessage so callers keep turnId/seq.
- db/chunks.ts: rekeyChunks(from,to) relocates a tab's full history to a
  backup tab (reversible — nothing is deleted).
- AgentEvent: compaction-started / -complete / -error variants.

api:
- AgentManager.compactTab(tempTabId, sourceTabId): side-effect-free
  resolveConnection() for the compactor model (configured compaction_model_*,
  else the source tab's own key+model), one-shot tool-less summary generation
  via a transient Agent, then relocate full history to a fresh backup tab and
  re-seed the canonical source id with [summary turn + preserved tail]. Source
  tab is locked (messages queue) during the run; queue drains afterward.
- Routes: POST /tabs/:id/compact, GET/PUT /tabs/settings/compaction-model.

frontend:
- "Compact conversation" button in ModelSelector (Chat Settings), between
  Working Directory and the agent toggle; idle-gated.
- Compaction-model key+model selector in Settings, beside the title model.
- Transient placeholder tab shows a large, non-faded "Please wait, compacting
  conversation…" screen; closing it cancels. Source input locked while running.
- Handle compaction-* events: reload compacted source, insert backup tab,
  refocus source, discard placeholder.

tests: core compaction unit tests, rekeyChunks DB test, AgentManager.compactTab
orchestration tests, and compaction route tests. All green (713 tests), biome
clean, all typechecks pass, frontend builds.

1 files changed, 54 insertions, 3 deletions

diff --git a/packages/api/src/routes/tabs.ts b/packages/api/src/routes/tabs.ts
index 28a89f1..2ae60ed 100644
--- a/packages/api/src/routes/tabs.ts
+++ b/packages/api/src/routes/tabs.ts
@@ -19,11 +19,18 @@ import { Hono } from "hono";
 
 export const tabsRoutes = new Hono();
 
-let getAgentManager: () => { stopTab(id: string): void; deleteTab(id: string): void } | null = () =>
-	null;
+let getAgentManager: () => {
+	stopTab(id: string): void;
+	deleteTab(id: string): void;
+	compactTab(tempTabId: string, sourceTabId: string): Promise<void>;
+} | null = () => null;
 
 export function setTabsAgentManager(
-	getter: () => { stopTab(id: string): void; deleteTab(id: string): void } | null,
+	getter: () => {
+		stopTab(id: string): void;
+		deleteTab(id: string): void;
+		compactTab(tempTabId: string, sourceTabId: string): Promise<void>;
+	} | null,
 ): void {
 	getAgentManager = getter;
 }
@@ -64,6 +71,28 @@ tabsRoutes.put("/settings/title-model", async (c) => {
 	return c.json({ success: true });
 });
 
+// Conversation-compaction model (key+model used to generate the summary).
+// Mirrors the title-model setting. When unset, compaction falls back to the
+// source tab's own key+model.
+tabsRoutes.get("/settings/compaction-model", (c) => {
+	const keyId = getSetting("compaction_model_key_id");
+	const modelId = getSetting("compaction_model_id");
+	return c.json({ keyId, modelId });
+});
+
+tabsRoutes.put("/settings/compaction-model", async (c) => {
+	const body = await c.req.json<{ keyId?: string | null; modelId?: string | null }>();
+	if (body.keyId !== undefined) {
+		if (body.keyId) setSetting("compaction_model_key_id", body.keyId);
+		else deleteSetting("compaction_model_key_id");
+	}
+	if (body.modelId !== undefined) {
+		if (body.modelId) setSetting("compaction_model_id", body.modelId);
+		else deleteSetting("compaction_model_id");
+	}
+	return c.json({ success: true });
+});
+
 // Reorder open tabs. Body `{ ids }` is the new left-to-right order of tab ids;
 // each tab's `position` is rewritten to its index. Must be declared before the
 // `/:id` routes so "reorder" isn't captured as an id param.
@@ -134,6 +163,28 @@ tabsRoutes.get("/:id/chunks", (c) => {
 	return c.json({ chunks, total, oldestSeq });
 });
 
+// Trigger conversation compaction. The `:id` is the TRANSIENT placeholder tab
+// hosting the "compacting…" UI; `sourceTabId` (body) is the conversation being
+// compacted. Fire-and-forget on the server: progress/outcome is delivered via
+// the `compaction-*` WS events. Returns 202 once the run is kicked off.
+tabsRoutes.post("/:id/compact", async (c) => {
+	const tempTabId = c.req.param("id");
+	const body = await c.req
+		.json<{ sourceTabId?: string }>()
+		.catch(() => ({}) as { sourceTabId?: string });
+	const sourceTabId = body.sourceTabId;
+	if (!sourceTabId || typeof sourceTabId !== "string") {
+		return c.json({ error: "sourceTabId is required" }, 400);
+	}
+	const mgr = getAgentManager();
+	if (!mgr) return c.json({ error: "agent manager unavailable" }, 503);
+	// Run in the background; outcome is emitted over WS.
+	void mgr.compactTab(tempTabId, sourceTabId).catch((err) => {
+		console.error(`[dispatch] compactTab error for ${sourceTabId}:`, err);
+	});
+	return c.json({ success: true }, 202);
+});
+
 tabsRoutes.patch("/:id", async (c) => {
 	const id = c.req.param("id");
 	const body = await c.req.json<{