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.

1 files changed, 2 insertions, 0 deletions

diff --git a/packages/transport-http/src/index.ts b/packages/transport-http/src/index.ts
index 244bce6..7165c55 100644
--- a/packages/transport-http/src/index.ts
+++ b/packages/transport-http/src/index.ts
@@ -30,6 +30,7 @@ export type {
 	CredentialStore,
 	LspService,
 	SessionOrchestrator,
+	SystemPromptService,
 	WarmService,
 } from "./seam.js";
 export {
@@ -39,4 +40,5 @@ export {
 	isValidWorkspaceSlug,
 	lspServiceHandle,
 	sessionOrchestratorHandle,
+	systemPromptHandle,
 } from "./seam.js";