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, 11 insertions, 1 deletions

diff --git a/bun.lock b/bun.lock
index 1dbb882..e6da398 100644
--- a/bun.lock
+++ b/bun.lock
@@ -189,6 +189,14 @@
         "@dispatch/ui-contract": "workspace:*",
       },
     },
+    "packages/system-prompt": {
+      "name": "@dispatch/system-prompt",
+      "version": "0.0.0",
+      "dependencies": {
+        "@dispatch/kernel": "workspace:*",
+        "@dispatch/transport-contract": "workspace:*",
+      },
+    },
     "packages/throughput-store": {
       "name": "@dispatch/throughput-store",
       "version": "0.0.0",
@@ -260,7 +268,7 @@
     },
     "packages/transport-contract": {
       "name": "@dispatch/transport-contract",
-      "version": "0.16.0",
+      "version": "0.18.0",
       "dependencies": {
         "@dispatch/ui-contract": "workspace:*",
         "@dispatch/wire": "workspace:*",
@@ -357,6 +365,8 @@
 
     "@dispatch/surface-registry": ["@dispatch/surface-registry@workspace:packages/surface-registry"],
 
+    "@dispatch/system-prompt": ["@dispatch/system-prompt@workspace:packages/system-prompt"],
+
     "@dispatch/throughput-store": ["@dispatch/throughput-store@workspace:packages/throughput-store"],
 
     "@dispatch/todo": ["@dispatch/todo@workspace:packages/todo"],