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, 23 insertions, 4 deletions

diff --git a/packages/frontend/src/lib/components/ChatPanel.svelte b/packages/frontend/src/lib/components/ChatPanel.svelte
index abdec17..ee1b8ef 100644
--- a/packages/frontend/src/lib/components/ChatPanel.svelte
+++ b/packages/frontend/src/lib/components/ChatPanel.svelte
@@ -10,6 +10,11 @@ let isLoadingMore = $state(false);
 
 const renderGroups = $derived(tabStore.activeTab?.renderGroups ?? []);
 const activeTabId = $derived(tabStore.activeTab?.id);
+// Compaction placeholder state for the active tab. `compactingSource` is set on
+// a transient placeholder tab while a conversation is being compacted;
+// `compactionError` is set if it failed.
+const compactingSource = $derived(tabStore.activeTab?.compactingSource ?? null);
+const compactionError = $derived(tabStore.activeTab?.compactionError ?? null);
 
 // Stable, turn-scoped render keys. A bubble's identity is `${turnId}:${role}:${n}`
 // (n = its index among same-(turn,role) messages) rather than the underlying
@@ -138,14 +143,28 @@ $effect(() => {
 			{#if isLoadingMore}
 				<div class="text-center text-xs text-base-content/40 py-2">Loading earlier messages...</div>
 			{/if}
-			{#if renderGroups.length === 0}
+			{#if compactingSource || compactionError}
+				<div class="flex flex-col items-center justify-center h-full gap-4 px-6 text-center">
+					{#if compactionError}
+						<div class="text-2xl font-semibold text-error">Compaction failed</div>
+						<div class="text-base text-base-content/70 max-w-md">{compactionError}</div>
+						<div class="text-sm text-base-content/50">Close this tab to dismiss — your conversation was not changed.</div>
+					{:else}
+						<span class="loading loading-spinner loading-lg text-primary"></span>
+						<div class="text-2xl font-semibold text-base-content">Please wait, compacting conversation…</div>
+						<div class="text-sm text-base-content/50">You can cancel by closing this tab.</div>
+					{/if}
+				</div>
+			{:else if renderGroups.length === 0}
 				<div class="flex items-center justify-center h-full text-base-content/40 text-sm">
 					Send a message to start a conversation
 				</div>
 			{/if}
-			{#each keyedMessages as { m, key } (key)}
-				<ChatMessageComponent message={m} tabId={activeTabId} />
-			{/each}
+			{#if !compactingSource && !compactionError}
+				{#each keyedMessages as { m, key } (key)}
+					<ChatMessageComponent message={m} tabId={activeTabId} />
+				{/each}
+			{/if}
 		</div>
 
 		<!-- Scroll-to-bottom button -->