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, 39 insertions, 0 deletions

diff --git a/packages/system-prompt/src/types.ts b/packages/system-prompt/src/types.ts
new file mode 100644
index 0000000..9e87512
--- /dev/null
+++ b/packages/system-prompt/src/types.ts
@@ -0,0 +1,39 @@
+/**
+ * System-prompt service handle + interface.
+ *
+ * The service is the single-responder anchor the session-orchestrator obtains
+ * (lazily, via `host.getService(systemPromptHandle)`) to construct and read the
+ * per-conversation resolved system prompt.
+ */
+import { defineService, type ServiceHandle } from "@dispatch/kernel";
+
+/**
+ * The system-prompt service.
+ *
+ * `construct` resolves the template once (first turn / compaction) and persists
+ * the result; `get` reads the persisted result on subsequent turns (cache-safe —
+ * no per-turn reconstruction).
+ */
+export interface SystemPromptService {
+	/**
+	 * Resolve the template against the current environment and persist the
+	 * result under `resolved:<conversationId>`. Returns the resolved string.
+	 * When no template is stored, the built-in default template is used. An
+	 * empty template yields an empty string.
+	 */
+	construct(
+		conversationId: string,
+		cwd: string,
+		context?: { readonly model?: string },
+	): Promise<string>;
+
+	/** Read the persisted resolved system prompt, or `null` if never constructed. */
+	get(conversationId: string): Promise<string | null>;
+}
+
+/**
+ * Typed handle anchoring the system-prompt service. The single symbol the
+ * session-orchestrator imports to reach the builder — no string-keyed lookup.
+ */
+export const systemPromptHandle: ServiceHandle<SystemPromptService> =
+	defineService<SystemPromptService>("system-prompt");