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/bun.lock b/bun.lock
index e6da398..ce128e4 100644
--- a/bun.lock
+++ b/bun.lock
@@ -155,6 +155,7 @@
         "@dispatch/credential-store": "workspace:*",
         "@dispatch/kernel": "workspace:*",
         "@dispatch/message-queue": "workspace:*",
+        "@dispatch/system-prompt": "workspace:*",
       },
     },
     "packages/skills": {
@@ -283,6 +284,7 @@
         "@dispatch/kernel": "workspace:*",
         "@dispatch/lsp": "workspace:*",
         "@dispatch/session-orchestrator": "workspace:*",
+        "@dispatch/system-prompt": "workspace:*",
         "@dispatch/throughput-store": "workspace:*",
         "@dispatch/transport-contract": "workspace:*",
         "hono": "^4.0.0",