feat(coder): orchestrator advanced flow patterns

- TriggerRule type (all_success/one_success/all_done) for parallel deps
- Variable substitution ($stepId.output.field) in agent step prompts
- Approval gate step kind (pauses flow via permission frames)
- flow_step_events table for append-only event-sourced step log
- evaluateTriggerRule pure function in flow-runner-decisions

apps/coder/src/services/flow-runner.ts

@@ -378,7 +378,8 @@ export function createFlowRunner(deps: Deps): FlowRunner {
     // flow's step.run already bakes in the evidence/YAGNI contracts.
     const persona = step.agent ? await loadPersona(step.agent) : '';
     const taskPrompt = await step.run(ctx);
-    const fullPrompt = persona ? `${persona}\n\n---\n\n${taskPrompt}` : taskPrompt;
+    const resolvedPrompt = resolveVariables(taskPrompt, ctx.results);
+    const fullPrompt = persona ? `${persona}\n\n---\n\n${resolvedPrompt}` : resolvedPrompt;
 
     // READ-ONLY (D-4): agent='qwen', mode_id='plan' are hardcoded, never
     // user-overridable. The dispatcher's qwen+plan rule forces the PTY hard gate.
@@ -763,3 +764,25 @@ export function createFlowRunner(deps: Deps): FlowRunner {
 function errMsg(e: unknown): string {
   return e instanceof Error ? e.message : String(e);
 }
+
+// ─── Variable substitution ───────────────────────────────────────────────────
+
+const VAR_PATTERN = /\$(\w+)\.output(?:\.(\w+(?:\.\w+)*))?/g;
+
+export function resolveVariables(prompt: string, results: Record<string, string>): string {
+  return prompt.replace(VAR_PATTERN, (match, stepId, fieldPath) => {
+    const output = results[stepId];
+    if (!output) return match;
+    if (!fieldPath) return output;
+    try {
+      const lines = output.split('\n');
+      for (const line of lines) {
+        const parsed = line.match(new RegExp(`^${fieldPath}:\\s*(.+)$`, 'i'));
+        if (parsed) return parsed[1]!.trim();
+      }
+    } catch {
+      // fall through
+    }
+    return match;
+  });
+}