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, 89 insertions, 0 deletions

diff --git a/packages/frontend/src/lib/components/SettingsPanel.svelte b/packages/frontend/src/lib/components/SettingsPanel.svelte
index 7a810d5..8b28957 100644
--- a/packages/frontend/src/lib/components/SettingsPanel.svelte
+++ b/packages/frontend/src/lib/components/SettingsPanel.svelte
@@ -26,6 +26,10 @@ function selectTheme(theme: Theme): void {
 let titleKeyId = $state<string | null>(null);
 let titleModelId = $state<string | null>(null);
 let availableModels = $state<string[]>([]);
+let compactionKeyId = $state<string | null>(null);
+let compactionModelId = $state<string | null>(null);
+let compactionModels = $state<string[]>([]);
+let loadingCompactionModels = $state(false);
 let loadingModels = $state(false);
 let autoExpandThinking = $state(appSettings.autoExpandThinking);
 let localChunkLimit = $state(appSettings.chunkLimit);
@@ -132,6 +136,19 @@ async function loadSettings(): Promise<void> {
 		// ignore
 	}
 	try {
+		const res = await fetch(`${apiBase}/tabs/settings/compaction-model`);
+		if (res.ok) {
+			const data = (await res.json()) as { keyId: string | null; modelId: string | null };
+			compactionKeyId = data.keyId;
+			if (compactionKeyId) {
+				await loadCompactionModelsForKey(compactionKeyId);
+			}
+			compactionModelId = data.modelId;
+		}
+	} catch {
+		// ignore
+	}
+	try {
 		const res = await fetch(`${apiBase}/tabs/settings/auto-expand-thinking`);
 		if (res.ok) {
 			const data = (await res.json()) as { value: string | null };
@@ -319,6 +336,48 @@ async function onModelChange(e: Event): Promise<void> {
 	saveTitleModel();
 }
 
+async function loadCompactionModelsForKey(keyId: string): Promise<void> {
+	loadingCompactionModels = true;
+	try {
+		const res = await fetch(`${apiBase}/models/available?keyId=${encodeURIComponent(keyId)}`);
+		if (!res.ok) {
+			compactionModels = [];
+			return;
+		}
+		const data = (await res.json()) as { models: string[] };
+		compactionModels = data.models ?? [];
+	} catch {
+		compactionModels = [];
+	} finally {
+		loadingCompactionModels = false;
+	}
+}
+
+function saveCompactionModel(): void {
+	fetch(`${apiBase}/tabs/settings/compaction-model`, {
+		method: "PUT",
+		headers: { "Content-Type": "application/json" },
+		body: JSON.stringify({ keyId: compactionKeyId, modelId: compactionModelId }),
+	}).catch(() => {});
+}
+
+async function onCompactionKeyChange(e: Event): Promise<void> {
+	const select = e.target as HTMLSelectElement;
+	compactionKeyId = select.value || null;
+	compactionModelId = null;
+	compactionModels = [];
+	if (compactionKeyId) {
+		await loadCompactionModelsForKey(compactionKeyId);
+	}
+	saveCompactionModel();
+}
+
+function onCompactionModelChange(e: Event): void {
+	const select = e.target as HTMLSelectElement;
+	compactionModelId = select.value || null;
+	saveCompactionModel();
+}
+
 $effect(() => {
 	void loadSettings();
 });
@@ -374,6 +433,36 @@ $effect(() => {
 
 		<div class="divider my-0"></div>
 
+		<p class="text-xs text-base-content/70">Conversation Compaction Model</p>
+		<p class="text-xs text-base-content/40">Used to summarize a conversation when you compact it. If unset, the tab's own key/model is used.</p>
+
+		<label class="text-xs text-base-content/60">
+			Key
+			<select class="select select-bordered select-sm w-full" onchange={onCompactionKeyChange} value={compactionKeyId ?? ""}>
+				<option value="">Select a key...</option>
+				{#each keys as key (key.id)}
+					<option value={key.id}>{key.id} ({key.provider})</option>
+				{/each}
+			</select>
+		</label>
+
+		<label class="text-xs text-base-content/60">
+			Model
+			<select
+				class="select select-bordered select-sm w-full"
+				onchange={onCompactionModelChange}
+				value={compactionModelId ?? ""}
+				disabled={!compactionKeyId || loadingCompactionModels}
+			>
+				<option value="">{loadingCompactionModels ? "Loading models..." : "Select a model..."}</option>
+				{#each compactionModels as model (model)}
+					<option value={model}>{model}</option>
+				{/each}
+			</select>
+		</label>
+
+		<div class="divider my-0"></div>
+
 		<p class="text-xs text-base-content/70">Chat</p>
 		<label class="flex items-center gap-2 cursor-pointer">
 			<input