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.

1 files changed, 10 insertions, 7 deletions

diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index e33ad2f..9fe7550 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -29,16 +29,19 @@ export {
 } from "./config/index.js";
 // Credentials
 export * from "./credentials/index.js";
-// Database
-export { closeDatabase, getDatabase, getDatabasePath } from "./db/index.js";
 export {
-	appendMessage,
-	clearMessagesForTab,
+	appendChunks,
+	clearChunksForTab,
+	explodeTurn,
+	explodeUserText,
+	getChunksForTab,
 	getMessagesForTab,
-	getTotalMessageCount,
+	getTotalChunkCount,
+	groupRowsToMessages,
 	type MessageRow,
-	updateMessage,
-} from "./db/messages.js";
+} from "./db/chunks.js";
+// Database
+export { closeDatabase, getDatabase, getDatabasePath } from "./db/index.js";
 export { deleteSetting, getSetting, setSetting } from "./db/settings.js";
 // Tabs & Messages
 export {