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.

2 files changed, 52 insertions, 1 deletions

diff --git a/packages/transport-contract/package.json b/packages/transport-contract/package.json
index 95e1aef..542ed90 100644
--- a/packages/transport-contract/package.json
+++ b/packages/transport-contract/package.json
@@ -1,6 +1,6 @@
 {
 	"name": "@dispatch/transport-contract",
-	"version": "0.17.0",
+	"version": "0.18.0",
 	"type": "module",
 	"private": true,
 	"main": "dist/index.js",
diff --git a/packages/transport-contract/src/index.ts b/packages/transport-contract/src/index.ts
index b195171..2a3bb9f 100644
--- a/packages/transport-contract/src/index.ts
+++ b/packages/transport-contract/src/index.ts
@@ -299,6 +299,57 @@ export interface CloseConversationResponse {
 	readonly abortedTurn: boolean;
 }
 
+// ─── System prompt template ───────────────────────────────────────────────────
+
+/**
+ * Response of `GET /system-prompt` — the current global system prompt template.
+ *
+ * The template is a text string with variable placeholders (`[type:name]`) and
+ * conditional blocks (`[if]`/`[else]`/`[endif]`). At construction time (first
+ * turn or compaction), variables are resolved against the conversation's cwd
+ * and system state. The resolved system prompt is persisted per conversation
+ * and reused on all subsequent turns (cache-safe — no per-turn reconstruction).
+ */
+export interface SystemPromptTemplateResponse {
+	/** The template text (may be empty — then no system prompt is sent). */
+	readonly template: string;
+}
+
+/**
+ * Body of `PUT /system-prompt` — set the global system prompt template.
+ *
+ * Changing the template does NOT affect existing conversations until they are
+ * compacted (the persisted resolved system prompt is stable). New
+ * conversations use the new template on their first turn.
+ */
+export interface SetSystemPromptTemplateRequest {
+	readonly template: string;
+}
+
+/**
+ * One available variable for the system prompt template, as reported by
+ * `GET /system-prompt/variables` so the frontend can render the variable
+ * selector buttons.
+ */
+export interface SystemPromptVariable {
+	/** The variable type/source: `"system"`, `"file"`, `"prompt"`, `"git"`. */
+	readonly type: string;
+	/** The variable name (e.g. `"time"`, `"date"`, `"os"`). For dynamic types, a description. */
+	readonly name: string;
+	/** Human-readable description of what the variable resolves to. */
+	readonly description: string;
+	/**
+	 * When `true`, any name is valid for this type (e.g. `file:<path>` accepts
+	 * any file path). The frontend should allow free-text input for the name.
+	 */
+	readonly dynamic?: boolean;
+}
+
+/** Response of `GET /system-prompt/variables`. */
+export interface SystemPromptVariablesResponse {
+	readonly variables: readonly SystemPromptVariable[];
+}
+
 // ─── Message queue (steering) ─────────────────────────────────────────────────
 
 /**