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, 179 insertions, 0 deletions

diff --git a/packages/core/tests/db/chunks.test.ts b/packages/core/tests/db/chunks.test.ts
new file mode 100644
index 0000000..fe54628
--- /dev/null
+++ b/packages/core/tests/db/chunks.test.ts
@@ -0,0 +1,179 @@
+import { describe, expect, it } from "vitest";
+import { explodeTurn, explodeUserText, groupRowsToMessages } from "../../src/chunks/transform.js";
+import type { Chunk, ChunkRow, ChunkRowDraft } from "../../src/types/index.js";
+
+// These tests cover the pure explode/group transforms — the heart of the flat
+// chunk-log storage model. No DB is required.
+
+/** Promote drafts to rows with synthetic seq/id/createdAt (as appendChunks would). */
+function toRows(drafts: ChunkRowDraft[], tabId = "tab-1", startSeq = 0): ChunkRow[] {
+	return drafts.map((d, i) => ({
+		id: `c${i}`,
+		tabId,
+		seq: startSeq + i,
+		turnId: d.turnId,
+		step: d.step,
+		role: d.role,
+		type: d.type,
+		data: d.data,
+		createdAt: 1000 + i,
+	}));
+}
+
+describe("explodeTurn", () => {
+	it("splits a tool-batch into separate tool_call (assistant) and tool_result (tool) rows", () => {
+		const chunks: Chunk[] = [
+			{ type: "thinking", text: "hmm", metadata: { anthropic: { signature: "S" } } },
+			{ type: "text", text: "let me read" },
+			{
+				type: "tool-batch",
+				calls: [
+					{ id: "a1", name: "read_file", arguments: { path: "x" }, result: "X", isError: false },
+					{ id: "a2", name: "read_file", arguments: { path: "y" }, result: "Y", isError: false },
+				],
+			},
+		];
+		const drafts = explodeTurn("turn-1", chunks);
+
+		// thinking, text, tool_call×2 (assistant), tool_result×2 (tool)
+		expect(drafts.map((d) => `${d.role}/${d.type}`)).toEqual([
+			"assistant/thinking",
+			"assistant/text",
+			"assistant/tool_call",
+			"assistant/tool_call",
+			"tool/tool_result",
+			"tool/tool_result",
+		]);
+		// All in the same step (one round-trip).
+		expect(drafts.every((d) => d.step === 0)).toBe(true);
+		expect(drafts.every((d) => d.turnId === "turn-1")).toBe(true);
+	});
+
+	it("increments step after each tool-batch (multi-step turn)", () => {
+		const chunks: Chunk[] = [
+			{ type: "text", text: "s0" },
+			{ type: "tool-batch", calls: [{ id: "a", name: "t", arguments: {}, result: "r" }] },
+			{ type: "text", text: "s1" },
+			{ type: "tool-batch", calls: [{ id: "b", name: "t", arguments: {}, result: "r" }] },
+			{ type: "text", text: "final" },
+		];
+		const drafts = explodeTurn("turn-1", chunks);
+		const byStep = (s: number) => drafts.filter((d) => d.step === s).map((d) => d.type);
+		expect(byStep(0)).toEqual(["text", "tool_call", "tool_result"]);
+		expect(byStep(1)).toEqual(["text", "tool_call", "tool_result"]);
+		expect(byStep(2)).toEqual(["text"]); // trailing final-step text, no tool-batch
+	});
+
+	it("omits tool_result rows for calls without a result", () => {
+		const chunks: Chunk[] = [
+			{ type: "tool-batch", calls: [{ id: "a", name: "t", arguments: {} }] },
+		];
+		const drafts = explodeTurn("turn-1", chunks);
+		expect(drafts.map((d) => d.type)).toEqual(["tool_call"]);
+	});
+});
+
+describe("groupRowsToMessages (round-trip)", () => {
+	it("reconstructs a user message then an assistant message with a per-step tool-batch", () => {
+		const rows = [
+			...toRows(explodeUserText("turn-1", "hello"), "tab-1", 0),
+			...toRows(
+				explodeTurn("turn-1", [
+					{ type: "text", text: "reading" },
+					{
+						type: "tool-batch",
+						calls: [
+							{
+								id: "a1",
+								name: "read_file",
+								arguments: { path: "x" },
+								result: "X",
+								isError: false,
+							},
+						],
+					},
+					{ type: "text", text: "done" },
+				]),
+				"tab-1",
+				1,
+			),
+		];
+
+		const msgs = groupRowsToMessages(rows);
+		expect(msgs.map((m) => m.role)).toEqual(["user", "assistant"]);
+		expect(msgs[0]?.chunks).toEqual([{ type: "text", text: "hello" }]);
+
+		const a = msgs[1];
+		if (!a) throw new Error("no assistant message");
+		// reconstructed: text, tool-batch(step0), text(step1)
+		expect(a.chunks.map((c) => c.type)).toEqual(["text", "tool-batch", "text"]);
+		const batch = a.chunks.find((c) => c.type === "tool-batch");
+		if (batch?.type !== "tool-batch") throw new Error("no batch");
+		expect(batch.calls[0]).toMatchObject({
+			id: "a1",
+			name: "read_file",
+			arguments: { path: "x" },
+			result: "X",
+			isError: false,
+		});
+	});
+
+	it("keeps each step's tool calls in its own tool-batch chunk", () => {
+		const rows = toRows(
+			explodeTurn("turn-1", [
+				{ type: "tool-batch", calls: [{ id: "a", name: "t", arguments: {}, result: "ra" }] },
+				{ type: "tool-batch", calls: [{ id: "b", name: "t", arguments: {}, result: "rb" }] },
+			]),
+		);
+		const msgs = groupRowsToMessages(rows);
+		expect(msgs).toHaveLength(1);
+		const batches = msgs[0]?.chunks.filter((c) => c.type === "tool-batch") ?? [];
+		expect(batches).toHaveLength(2);
+	});
+
+	it("round-trips a multi-step assistant turn back to its original chunk shape", () => {
+		const original: Chunk[] = [
+			{ type: "thinking", text: "plan", metadata: { anthropic: { signature: "S" } } },
+			{ type: "text", text: "step0" },
+			{
+				type: "tool-batch",
+				calls: [
+					{ id: "a", name: "read_file", arguments: { path: "p" }, result: "R", isError: false },
+				],
+			},
+			{ type: "text", text: "final" },
+		];
+		const rows = toRows(explodeTurn("turn-1", original));
+		const msgs = groupRowsToMessages(rows);
+		expect(msgs).toHaveLength(1);
+		expect(msgs[0]?.chunks).toEqual(original);
+	});
+
+	it("tolerates an orphan tool_result whose tool_call was paged out", () => {
+		const rows = toRows([
+			{
+				turnId: "turn-1",
+				step: 0,
+				role: "tool",
+				type: "tool_result",
+				data: { callId: "z", name: "t", result: "R", isError: false },
+			},
+		]);
+		const msgs = groupRowsToMessages(rows);
+		expect(msgs).toHaveLength(1);
+		const batch = msgs[0]?.chunks[0];
+		if (batch?.type !== "tool-batch") throw new Error("no batch");
+		expect(batch.calls[0]).toMatchObject({ id: "z", result: "R" });
+	});
+
+	it("breaks the assistant grouping on a user or system row", () => {
+		const rows = [
+			...toRows(explodeUserText("t1", "q1"), "tab", 0),
+			...toRows(explodeTurn("t1", [{ type: "text", text: "a1" }]), "tab", 1),
+			...toRows(explodeUserText("t2", "q2"), "tab", 2),
+			...toRows(explodeTurn("t2", [{ type: "system", kind: "notice", text: "n" }]), "tab", 3),
+		];
+		const msgs = groupRowsToMessages(rows);
+		expect(msgs.map((m) => m.role)).toEqual(["user", "assistant", "user", "system"]);
+	});
+});