refactor(chunks): append-only chunk log with per-step cache-stable wire

Replace the message-as-container model with a flat, append-only chunk log.

- chunks table (id, tab_id, seq, turn_id, step, role, type, data_json): one
  row per chunk; tool_call (assistant) and tool_result (tool) are SEPARATE
  rows linked by callId. Message/turn are derived groupings, not stored.
- chunks/transform.ts: DB-free explode (Chunk[] -> rows) / group (rows ->
  messages), shared by backend and the browser frontend.
- Cache fix: toModelMessages segments each turn at tool-batch boundaries into
  stable [assistant, tool] pairs per step, so earlier steps serialize
  byte-identically across requests (kills the prompt-cache churn).
- agent-manager persists a turn's chunks on seal (once), discarding a failed
  fallback attempt's partial chunks; rebuilds agent history from the log.
- GET /messages windows the log by chunk seq then groups; loadMoreMessages
  merges a turn split across the window boundary by turnId.
- One-shot migration drops the legacy messages table and clears tabs;
  settings/credentials/keys/usage preserved.

Full suite green (317 tests); biome, tsc, and svelte-check clean.

2 files changed, 27 insertions, 4 deletions

diff --git a/packages/api/tests/agent-manager.test.ts b/packages/api/tests/agent-manager.test.ts
index ba14cad..6b016db 100644
--- a/packages/api/tests/agent-manager.test.ts
+++ b/packages/api/tests/agent-manager.test.ts
@@ -259,8 +259,15 @@ vi.mock("@dispatch/core", () => ({
 	getSetting(_key: string) {
 		return null;
 	},
-	appendMessage() {},
-	updateMessage() {},
+	appendChunks() {
+		return [];
+	},
+	explodeUserText() {
+		return [];
+	},
+	explodeTurn() {
+		return [];
+	},
 	getMessagesForTab(tabId: string) {
 		return fakeMessagesByTab.get(tabId) ?? [];
 	},
diff --git a/packages/api/tests/routes.test.ts b/packages/api/tests/routes.test.ts
index eba2226..f4de845 100644
--- a/packages/api/tests/routes.test.ts
+++ b/packages/api/tests/routes.test.ts
@@ -181,11 +181,27 @@ vi.mock("@dispatch/core", () => ({
 	getSetting(_key: string) {
 		return null;
 	},
-	appendMessage() {},
-	updateMessage() {},
+	appendChunks() {
+		return [];
+	},
+	explodeUserText() {
+		return [];
+	},
+	explodeTurn() {
+		return [];
+	},
 	getMessagesForTab() {
 		return [];
 	},
+	getChunksForTab() {
+		return [];
+	},
+	groupRowsToMessages() {
+		return [];
+	},
+	getTotalChunkCount() {
+		return 0;
+	},
 	appendEventToChunks(_chunks: unknown[], _event: unknown) {
 		// no-op stub
 	},