feat(hooks/atlas): add BOULDER_COMPLETE_PROMPT template and SessionState guard

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/hooks/atlas/system-reminder-templates.test.ts

@@ -1,5 +1,6 @@
 import { describe, it, expect } from "bun:test"
 import {
+  BOULDER_COMPLETE_PROMPT,
   BOULDER_CONTINUATION_PROMPT,
   SINGLE_TASK_DIRECTIVE,
   VERIFICATION_REMINDER,
@@ -47,6 +48,14 @@ describe("VERIFICATION_REMINDER", () => {
   })
 })
 
+describe("BOULDER_COMPLETE_PROMPT", () => {
+  it("contains the required placeholders", () => {
+    expect(BOULDER_COMPLETE_PROMPT).toContain("{PLAN_NAME}")
+    expect(BOULDER_COMPLETE_PROMPT).toContain("{ELAPSED_HUMAN}")
+    expect(BOULDER_COMPLETE_PROMPT).toContain("{TASK_BREAKDOWN}")
+  })
+})
+
 describe("VERIFICATION_REMINDER_GEMINI", () => {
   it("contains node_modules exclusion pathspec in git diff command", () => {
     expect(VERIFICATION_REMINDER_GEMINI).toContain(":!node_modules")