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, 11 insertions, 2 deletions

diff --git a/packages/frontend/src/lib/components/ChatInput.svelte b/packages/frontend/src/lib/components/ChatInput.svelte
index 079ef4a..f954be8 100644
--- a/packages/frontend/src/lib/components/ChatInput.svelte
+++ b/packages/frontend/src/lib/components/ChatInput.svelte
@@ -17,6 +17,13 @@ const inputValue = $derived(tabStore.activeTab?.draft ?? "");
 const cacheStats = $derived(tabStore.activeTab?.cacheStats ?? null);
 
 const isRunning = $derived(agentStatus === "running");
+// Lock input while this tab is mid-compaction: either it's the source
+// conversation being summarized, or it's the transient placeholder tab.
+const compactLocked = $derived(
+	(tabStore.activeTab?.isCompacting ?? false) ||
+		(tabStore.activeTab?.compactingSource ?? null) !== null ||
+		(tabStore.activeTab?.compactionError ?? null) !== null,
+);
 const hasText = $derived(inputValue.trim().length > 0);
 // While generating with an empty box, the primary action is "stop". With text
 // in the box, it stays "send" (the message is queued behind the live turn).
@@ -88,6 +95,7 @@ function handleKeydown(e: KeyboardEvent) {
 }
 
 function submit() {
+	if (compactLocked) return;
 	const text = inputValue.trim();
 	if (!text) return;
 	if (tabId) tabStore.setDraft(tabId, "");
@@ -110,7 +118,8 @@ function primaryAction() {
 			bind:this={inputEl}
 			value={inputValue}
 			rows="1"
-			placeholder="Type a message..."
+			placeholder={compactLocked ? "Compaction in progress…" : "Type a message..."}
+			disabled={compactLocked}
 			class="textarea textarea-ghost flex-1 resize-none leading-normal !min-h-0 h-auto"
 			onkeydown={handleKeydown}
 			oninput={handleInput}
@@ -120,7 +129,7 @@ function primaryAction() {
 		<button
 			type="button"
 			class="btn w-20 shrink-0 {showStop ? 'btn-error btn-outline' : 'btn-primary'}"
-			disabled={!showStop && !hasText}
+			disabled={compactLocked || (!showStop && !hasText)}
 			onclick={primaryAction}
 			title={showStop ? "Stop generation" : "Send message"}
 		>