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

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+---
+phase: 12-phase-task-decomposition
+plan: 05
+subsystem: api
+tags: [trpc, agent, prompts, decompose, spawn, architect]
+
+# Dependency graph
+requires:
+  - phase: 12-03
+    provides: handleDecomposeOutput method in ClaudeAgentManager
+  - phase: 12-04
+    provides: PlanRepository in tRPC context, Plan CRUD procedures
+provides:
+  - buildDecomposePrompt function for decompose mode agent prompts
+  - spawnArchitectDecompose tRPC procedure for spawning decompose agents
+affects: [cli-decompose-commands, 12-06, 12-08]
+
+# Tech tracking
+tech-stack:
+  added: []
+  patterns:
+    - "Decompose prompt follows breakdown prompt pattern with plan/phase context"
+    - "Architect spawn procedure validates entity existence before spawning"
+
+key-files:
+  created: []
+  modified:
+    - src/agent/prompts.ts
+    - src/agent/index.ts
+    - src/trpc/router.ts
+
+key-decisions:
+  - "buildDecomposePrompt takes Plan and Phase (not Initiative) to provide plan-level context"
+  - "spawnArchitectDecompose associates with planId (not initiativeId) matching decompose scope"
+
+patterns-established:
+  - "Architect mode prompts: discuss/breakdown work on Initiative, decompose works on Plan"
+
+# Metrics
+duration: 4min
+completed: 2026-02-01
+---
+
+# Phase 12 Plan 05: Decompose Prompts & Spawn Procedure Summary
+
+**Added buildDecomposePrompt function and spawnArchitectDecompose tRPC procedure for spawning architect agents in decompose mode**
+
+## Performance
+
+- **Duration:** 4 min
+- **Started:** 2026-02-01T10:46:30Z
+- **Completed:** 2026-02-01T10:50:38Z
+- **Tasks:** 3
+- **Files modified:** 3
+
+## Accomplishments
+
+- Created comprehensive buildDecomposePrompt function explaining task breakdown rules and output format
+- Exported buildDecomposePrompt from agent module public API
+- Added spawnArchitectDecompose tRPC procedure with plan/phase validation
+- All 402 tests pass with no regressions
+
+## Task Commits
+
+Each task was committed atomically:
+
+1. **Task 1: Create buildDecomposePrompt function** - `48336ec` (feat)
+2. **Task 2: Export buildDecomposePrompt from agent index** - `7ff979b` (feat)
+3. **Task 3: Add spawnArchitectDecompose tRPC procedure** - `1fcbdf5` (feat)
+
+## Files Created/Modified
+
+- `src/agent/prompts.ts` - Added buildDecomposePrompt function with Plan/Phase types import
+- `src/agent/index.ts` - Added buildDecomposePrompt to public exports
+- `src/trpc/router.ts` - Added spawnArchitectDecompose procedure and buildDecomposePrompt import
+
+## Decisions Made
+
+1. **12-05: buildDecomposePrompt signature** - Takes Plan and Phase parameters (not Initiative) since decompose operates at plan level, providing plan name/description and phase context
+2. **12-05: spawnArchitectDecompose taskId** - Associates spawned agent with planId (not initiativeId) to match the decompose scope
+
+## Deviations from Plan
+
+None - plan executed exactly as written.
+
+## Issues Encountered
+
+None
+
+## User Setup Required
+
+None - no external service configuration required.
+
+## Next Phase Readiness
+
+- Decompose prompts and spawn procedure ready for CLI integration
+- Ready for 12-06 (Decompose CLI Commands)
+- MockAgentManager already supports decompose_complete scenarios (from 12-03)
+- Full decompose workflow can now be invoked via tRPC
+
+---
+*Phase: 12-phase-task-decomposition*
+*Completed: 2026-02-01*