feat(system-prompt): template-based system prompt builder extension

New @dispatch/system-prompt extension (standard tier):
- Pure parser: [type:name] variables, [if]/[else]/[endif] conditionals,
  negated [if !...], nested blocks, unmatched-tag pass-through.
- Variable resolver (injected adapters): system:time/date/os/hostname,
  prompt:cwd/model/conversation_id, git:branch/status, file:<path> (dynamic).
- Service handle: construct (resolve+persist) + get (cached, cache-safe).
- Default template: persona + AGENTS.md if exists + cwd.
- 52 tests (parser 29, resolver 12, catalog 3, service 8).

transport-contract 0.17.0→0.18.0: SystemPromptTemplateResponse,
SetSystemPromptTemplateRequest, SystemPromptVariable, SystemPromptVariablesResponse.

Design: notes/system-prompt-design.md (caching constraint, compaction
integration, wave plan). 1384 vitest pass.

1 files changed, 151 insertions, 0 deletions

diff --git a/packages/system-prompt/src/service.test.ts b/packages/system-prompt/src/service.test.ts
new file mode 100644
index 0000000..91592f8
--- /dev/null
+++ b/packages/system-prompt/src/service.test.ts
@@ -0,0 +1,151 @@
+import type { StorageNamespace } from "@dispatch/kernel";
+import { describe, expect, it } from "vitest";
+import type { GitSpawnResult, ResolverAdapters, ResolverFs } from "./resolver.js";
+import { createSystemPromptService, DEFAULT_TEMPLATE } from "./service.js";
+
+/** In-memory StorageNamespace for tests. */
+function memoryStorage(): StorageNamespace {
+	const store = new Map<string, string>();
+	return {
+		get: async (key: string) => store.get(key) ?? null,
+		set: async (key: string, value: string) => {
+			store.set(key, value);
+		},
+		delete: async (key: string) => {
+			store.delete(key);
+		},
+		has: async (key: string) => store.has(key),
+		keys: async (prefix?: string) =>
+			[...store.keys()].filter((k) => (prefix === undefined ? true : k.startsWith(prefix))),
+	};
+}
+
+function fakeFs(files: ReadonlyMap<string, string>): ResolverFs {
+	return {
+		readText: async (path: string) => files.get(path) ?? "",
+		exists: async (path: string) => files.has(path),
+	};
+}
+
+const failSpawn = async (): Promise<GitSpawnResult> => ({
+	stdout: "",
+	stderr: "",
+	exitCode: 128,
+});
+
+function adapters(files: ReadonlyMap<string, string>): ResolverAdapters {
+	return {
+		spawn: failSpawn,
+		fs: fakeFs(files),
+		now: () => new Date("2024-06-15T12:30:00.000Z"),
+		platform: () => "linux",
+		hostname: () => "myhost",
+	};
+}
+
+describe("system-prompt service", () => {
+	it("construct persists and returns the resolved string", async () => {
+		// 14. construct writes to storage and returns the resolved string.
+		const storage = memoryStorage();
+		const service = createSystemPromptService({
+			storage,
+			adapters: adapters(new Map([["/proj/AGENTS.md", "RULES"]])),
+		});
+
+		const result = await service.construct("conv-1", "/proj", { model: "gpt-4" });
+
+		expect(result).toContain("You are a helpful coding assistant.");
+		expect(result).toContain("RULES");
+		expect(result).toContain("/proj");
+		// persisted under resolved:<conversationId>
+		expect(await storage.get("resolved:conv-1")).toBe(result);
+	});
+
+	it("get returns persisted value after construct", async () => {
+		// 15. after construct, get returns the same string.
+		const service = createSystemPromptService({
+			storage: memoryStorage(),
+			adapters: adapters(new Map()),
+		});
+
+		// before construct → null
+		expect(await service.get("conv-2")).toBeNull();
+
+		const result = await service.construct("conv-2", "/proj");
+		expect(await service.get("conv-2")).toBe(result);
+	});
+
+	it("get returns null before construct", async () => {
+		const service = createSystemPromptService({
+			storage: memoryStorage(),
+			adapters: adapters(new Map()),
+		});
+
+		expect(await service.get("never-constructed")).toBeNull();
+	});
+
+	it("empty/no template stored → default template → non-empty", async () => {
+		// 16. no template stored → default template used → resolves to non-empty.
+		const service = createSystemPromptService({
+			storage: memoryStorage(),
+			adapters: adapters(new Map()), // no AGENTS.md
+		});
+
+		const result = await service.construct("conv-3", "/proj");
+
+		expect(result.length).toBeGreaterThan(0);
+		expect(result).toContain("You are a helpful coding assistant.");
+		expect(result).toContain("/proj");
+		// no AGENTS.md file → the [if file:AGENTS.md] block is omitted
+		expect(result).not.toContain("AGENTS.md");
+	});
+
+	it("stored template is used instead of default", async () => {
+		const storage = memoryStorage();
+		await storage.set("template", "cwd=[prompt:cwd] os=[system:os]");
+		const service = createSystemPromptService({
+			storage,
+			adapters: adapters(new Map()),
+		});
+
+		const result = await service.construct("conv-4", "/work");
+		expect(result).toBe("cwd=/work os=linux");
+	});
+
+	it("empty stored template → empty string", async () => {
+		const storage = memoryStorage();
+		await storage.set("template", "");
+		const service = createSystemPromptService({
+			storage,
+			adapters: adapters(new Map()),
+		});
+
+		const result = await service.construct("conv-5", "/proj");
+		expect(result).toBe("");
+		expect(await service.get("conv-5")).toBe("");
+	});
+
+	it("construct is independent per conversation", async () => {
+		const storage = memoryStorage();
+		await storage.set("template", "[prompt:cwd]");
+		const service = createSystemPromptService({
+			storage,
+			adapters: adapters(new Map()),
+		});
+
+		const a = await service.construct("conv-a", "/dir-a");
+		const b = await service.construct("conv-b", "/dir-b");
+
+		expect(a).toBe("/dir-a");
+		expect(b).toBe("/dir-b");
+		expect(await service.get("conv-a")).toBe("/dir-a");
+		expect(await service.get("conv-b")).toBe("/dir-b");
+	});
+
+	it("DEFAULT_TEMPLATE contains the expected structure", () => {
+		expect(DEFAULT_TEMPLATE).toContain("You are a helpful coding assistant.");
+		expect(DEFAULT_TEMPLATE).toContain("[if file:AGENTS.md]");
+		expect(DEFAULT_TEMPLATE).toContain("[file:AGENTS.md]");
+		expect(DEFAULT_TEMPLATE).toContain("[prompt:cwd]");
+	});
+});