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, 150 insertions, 0 deletions

diff --git a/packages/core/src/db/chunks.ts b/packages/core/src/db/chunks.ts
new file mode 100644
index 0000000..6841eb5
--- /dev/null
+++ b/packages/core/src/db/chunks.ts
@@ -0,0 +1,150 @@
+import { randomUUID } from "node:crypto";
+import {
+	explodeTurn,
+	explodeUserText,
+	groupRowsToMessages,
+	type MessageRow,
+} from "../chunks/transform.js";
+import type { ChunkData, ChunkRow, ChunkRowDraft, TextData } from "../types/index.js";
+import { getDatabase } from "./index.js";
+
+// Re-export the DB-free transforms so existing barrel consumers
+// (`@dispatch/core`) keep importing them from here. The browser frontend deep-
+// imports them directly from `chunks/transform.js` to avoid the DB dependency.
+export { explodeTurn, explodeUserText, groupRowsToMessages, type MessageRow };
+
+// ─── Persistence ─────────────────────────────────────────────────
+
+function mapRow(row: Record<string, unknown>): ChunkRow {
+	let data: ChunkData;
+	try {
+		data = JSON.parse(row.data_json as string) as ChunkData;
+	} catch {
+		data = { text: "" } as TextData;
+	}
+	return {
+		id: row.id as string,
+		tabId: row.tab_id as string,
+		seq: row.seq as number,
+		turnId: row.turn_id as string,
+		step: row.step as number,
+		role: row.role as ChunkRow["role"],
+		type: row.type as ChunkRow["type"],
+		data,
+		createdAt: row.created_at as number,
+	};
+}
+
+/**
+ * Append one or more chunk-row drafts to a tab, assigning a monotonic per-tab
+ * `seq` and a fresh id/timestamp to each. Returns the inserted rows in order.
+ */
+export function appendChunks(tabId: string, drafts: ChunkRowDraft[]): ChunkRow[] {
+	if (drafts.length === 0) return [];
+	const db = getDatabase();
+	const maxSeq = db
+		.query("SELECT COALESCE(MAX(seq), -1) as max_seq FROM chunks WHERE tab_id = $tabId")
+		.get({ $tabId: tabId }) as { max_seq: number };
+	let seq = (maxSeq?.max_seq ?? -1) + 1;
+	const now = Date.now();
+	const insert = db.query(
+		`INSERT INTO chunks (id, tab_id, seq, turn_id, step, role, type, data_json, created_at)
+		 VALUES ($id, $tabId, $seq, $turnId, $step, $role, $type, $dataJson, $now)`,
+	);
+	const out: ChunkRow[] = [];
+	for (const draft of drafts) {
+		const id = randomUUID();
+		insert.run({
+			$id: id,
+			$tabId: tabId,
+			$seq: seq,
+			$turnId: draft.turnId,
+			$step: draft.step,
+			$role: draft.role,
+			$type: draft.type,
+			$dataJson: JSON.stringify(draft.data),
+			$now: now,
+		});
+		out.push({
+			id,
+			tabId,
+			seq,
+			turnId: draft.turnId,
+			step: draft.step,
+			role: draft.role,
+			type: draft.type,
+			data: draft.data,
+			createdAt: now,
+		});
+		seq++;
+	}
+	return out;
+}
+
+/**
+ * Read chunk rows for a tab in `seq` order (ASC). Pagination mirrors the old
+ * message pagination but at chunk granularity:
+ *   - no options → all rows;
+ *   - `before` → rows with `seq < before`, most-recent-first then reversed;
+ *   - `limit` → most recent `limit` rows, reversed to ASC.
+ */
+export function getChunksForTab(
+	tabId: string,
+	options?: { limit?: number; before?: number },
+): ChunkRow[] {
+	const db = getDatabase();
+	if (!options) {
+		const rows = db
+			.query("SELECT * FROM chunks WHERE tab_id = $tabId ORDER BY seq ASC")
+			.all({ $tabId: tabId }) as Array<Record<string, unknown>>;
+		return rows.map(mapRow);
+	}
+	const { limit, before } = options;
+	if (before !== undefined) {
+		if (limit !== undefined) {
+			const rows = db
+				.query(
+					"SELECT * FROM chunks WHERE tab_id = $tabId AND seq < $before ORDER BY seq DESC LIMIT $limit",
+				)
+				.all({ $tabId: tabId, $before: before, $limit: limit }) as Array<Record<string, unknown>>;
+			return rows.map(mapRow).reverse();
+		}
+		const rows = db
+			.query("SELECT * FROM chunks WHERE tab_id = $tabId AND seq < $before ORDER BY seq DESC")
+			.all({ $tabId: tabId, $before: before }) as Array<Record<string, unknown>>;
+		return rows.map(mapRow).reverse();
+	}
+	if (limit !== undefined) {
+		const rows = db
+			.query("SELECT * FROM chunks WHERE tab_id = $tabId ORDER BY seq DESC LIMIT $limit")
+			.all({ $tabId: tabId, $limit: limit }) as Array<Record<string, unknown>>;
+		return rows.map(mapRow).reverse();
+	}
+	const rows = db
+		.query("SELECT * FROM chunks WHERE tab_id = $tabId ORDER BY seq ASC")
+		.all({ $tabId: tabId }) as Array<Record<string, unknown>>;
+	return rows.map(mapRow);
+}
+
+/**
+ * Derived, grouped view of a tab's full history as messages. Used to
+ * pre-populate the agent's in-memory `ChatMessage[]` history when an Agent is
+ * (re)constructed. Always reads the full log (grouping a partial window would
+ * be lossy for the rebuild path).
+ */
+export function getMessagesForTab(tabId: string): MessageRow[] {
+	return groupRowsToMessages(getChunksForTab(tabId));
+}
+
+export function getTotalChunkCount(tabId: string): number {
+	const db = getDatabase();
+	const row = db
+		.query("SELECT COUNT(*) as count FROM chunks WHERE tab_id = $tabId")
+		.get({ $tabId: tabId }) as { count: number } | null;
+	return row?.count ?? 0;
+}
+
+export function clearChunksForTab(tabId: string): void {
+	const db = getDatabase();
+	db.query("DELETE FROM chunks WHERE tab_id = $tabId").run({ $tabId: tabId });
+}