docs(12-05): complete Decompose Prompts & Spawn Procedure plan

Tasks completed: 3/3
- Create buildDecomposePrompt function
- Export buildDecomposePrompt from agent index
- Add spawnArchitectDecompose tRPC procedure

SUMMARY: .planning/phases/12-phase-task-decomposition/12-05-SUMMARY.md

.planning/STATE.md

@@ -10,11 +10,11 @@ See: .planning/PROJECT.md (updated 2026-01-31)
 ## Current Position
 
 Phase: 12 of 13 (Phase Task Decomposition)
-Plan: 4 of 8 in current phase
+Plan: 5 of 8 in current phase
 Status: In progress
-Last activity: 2026-02-01 — Completed 12-04-PLAN.md
+Last activity: 2026-02-01 — Completed 12-05-PLAN.md
 
-Progress: ██████░░░░ 61%
+Progress: ██████░░░░ 64%
 
 ## Performance Metrics
 
@@ -152,6 +152,8 @@ Recent decisions affecting current work:
 - 12-03: handleDecomposeOutput follows exact structure of handleBreakdownOutput for pattern consistency
 - 12-04: Extended TaskRepository with createDependency for hexagonal architecture compliance
 - 12-04: createTasksFromDecomposition maps task numbers to IDs for dependency creation
+- 12-05: buildDecomposePrompt takes Plan and Phase (not Initiative) to provide plan-level context
+- 12-05: spawnArchitectDecompose associates with planId (not initiativeId) matching decompose scope
 
 ### Pending Todos
 
@@ -172,5 +174,5 @@ None.
 ## Session Continuity
 
 Last session: 2026-02-01
-Stopped at: Completed 12-04-PLAN.md (Plan & Task tRPC Procedures)
+Stopped at: Completed 12-05-PLAN.md (Decompose Prompts & Spawn Procedure)
 Resume file: None