feat(coding-agent): added snapcompact savings journaling for imaged tool results

- Added a `snapcompact-savings.jsonl` append-only journal for per-tool savings records.
- Extended `SnapcompactInlineTransformer` to compute `savedTokens` and emit savings to an optional sink.
- Integrated `createSnapcompactSavingsRecorder` into `createAgentSession` for session-aware tool-result metrics.
- Skipped journaling entries without a session and deduped duplicate `toolCallId`s per recorder lifecycle.
- Added tests for savings-sink emissions and savings-journal read/write behavior.

packages/coding-agent/test/snapcompact-inline.test.ts

@@ -115,6 +115,40 @@ describe("SnapcompactInlineTransformer", () => {
 		expect(result.systemPrompt).toBe(context.systemPrompt);
 	});
 
+	it("reports per-tool-result savings to the sink for each imaged result only", () => {
+		const received: Array<{ toolCallId: string; savedTokens: number }>[] = [];
+		let model = "";
+		const transformer = new SnapcompactInlineTransformer(
+			{ renderSystemPrompt: "none", renderToolResults: true },
+			(savings, m) => {
+				received.push(savings.map(s => ({ ...s })));
+				model = m.id;
+			},
+		);
+		transformer.transform(makeContext(), makeModel());
+
+		// Only the large historical result (call_1) is imaged; call_2 is small,
+		// call_3 is the most-recent (kept crisp).
+		expect(received).toHaveLength(1);
+		expect(received[0]).toHaveLength(1);
+		expect(received[0][0].toolCallId).toBe("call_1");
+		expect(received[0][0].savedTokens).toBeGreaterThan(0);
+		expect(model).toBe("test-model");
+	});
+
+	it("never calls the savings sink when nothing is imaged", () => {
+		let calls = 0;
+		const transformer = new SnapcompactInlineTransformer(
+			{ renderSystemPrompt: "none", renderToolResults: true },
+			() => {
+				calls++;
+			},
+		);
+		// Text-only model → vision gate short-circuits before any swap.
+		transformer.transform(makeContext(), makeModel({ input: ["text"] }));
+		expect(calls).toBe(0);
+	});
+
 	it("never mutates the input context (persisted history shares these references)", () => {
 		const transformer = new SnapcompactInlineTransformer({ renderSystemPrompt: "all", renderToolResults: true });
 		const context = makeContext();