feat(system-prompt): wire into turn flow + compaction + API routes

session-orchestrator:
- Wire systemPromptService as optional dep (lazy via host.getService)
- Regular turn: construct on first turn (new conversation), get on subsequent
  turns, set on providerOpts.systemPrompt (cache-safe)
- Compaction: construct (fresh resolve) + append COMPACTION_SYSTEM_PROMPT
- 12 new tests (construct/get/service-unavailable/compaction)

transport-http:
- GET /system-prompt (returns template or DEFAULT_TEMPLATE)
- PUT /system-prompt (validate + setTemplate, 503 when unavailable)
- GET /system-prompt/variables (static catalog, always available)
- 6 new tests

system-prompt service: added getTemplate/setTemplate to interface + impl.

1396 vitest pass. typecheck + biome clean.

2 files changed, 15 insertions, 0 deletions

diff --git a/packages/system-prompt/src/service.ts b/packages/system-prompt/src/service.ts
index 45645b9..4b4436f 100644
--- a/packages/system-prompt/src/service.ts
+++ b/packages/system-prompt/src/service.ts
@@ -63,5 +63,14 @@ export function createSystemPromptService(deps: SystemPromptServiceDeps): System
 		async get(conversationId) {
 			return deps.storage.get(resolvedKey(conversationId));
 		},
+
+		async getTemplate() {
+			const stored = await deps.storage.get(TEMPLATE_KEY);
+			return stored ?? DEFAULT_TEMPLATE;
+		},
+
+		async setTemplate(template) {
+			await deps.storage.set(TEMPLATE_KEY, template);
+		},
 	};
 }
diff --git a/packages/system-prompt/src/types.ts b/packages/system-prompt/src/types.ts
index 9e87512..dd6aa69 100644
--- a/packages/system-prompt/src/types.ts
+++ b/packages/system-prompt/src/types.ts
@@ -29,6 +29,12 @@ export interface SystemPromptService {
 
 	/** Read the persisted resolved system prompt, or `null` if never constructed. */
 	get(conversationId: string): Promise<string | null>;
+
+	/** Read the global template (or `DEFAULT_TEMPLATE` when none is stored). */
+	getTemplate(): Promise<string>;
+
+	/** Set (upsert) the global template. An empty string means "no system prompt". */
+	setTemplate(template: string): Promise<void>;
 }
 
 /**