Add story documentation for CreatePrefabTool

Added detailed story documentation for the CreatePrefabTool feature including user story, acceptance criteria, technical notes, and validation checklist.

Author: CoderGamester

docs/create-prefab-tool-story.md

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+# CreatePrefabTool – Brownfield Addition
+
+## User Story
+
+As a **Unity game developer**,
+I want an **AI-driven CreatePrefabTool that automatically generates a prefab from a newly created C# `MonoBehaviour` script and updates its serialized fields**,
+So that **I can set up gameplay entities (e.g., player, enemy, spawner) rapidly without repetitive manual steps**.
+
+## Story Context
+
+**Existing System Integration:**
+- Integrates with: *Unity MCP* (C# Editor tools & Node.js server)
+- Technology: *Unity 2022.3+, TypeScript/Node 18+*
+- Follows pattern: *Existing MCP tool architecture (C# `McpToolBase` & TS wrapper)*
+- Touch points:
+  - `Editor/Tools/` → new `CreatePrefabTool.cs`
+  - `Server~/src/tools/` → new `createPrefabTool.ts`
+  - `Server~/src/index.ts` tool registration
+  - Asset path: `Assets/Prefabs/<name>.prefab`
+
+## Acceptance Criteria
+
+### Functional Requirements
+1. Given a fully-compiled C# script name and desired prefab name, invoking the MCP tool **creates** a prefab under `Assets/Prefabs/` with the script component attached.
+2. Tool accepts an optional JSON object of *serialized field values* and applies them to the prefab instance.
+3. Tool returns the prefab asset path on success.
+
+### Integration Requirements
+4. Existing MCP tools continue to operate unchanged.
+5. New tool conforms to standard MCP JSON request/response schema.
+6. Prefab creation works both via CLI (`stdio` transport) and future Web transport.
+
+### Quality Requirements
+7. No console errors/warnings after prefab generation.
+8. Prefab asset passes Unity import/refresh without issues.
+9. Documentation for the tool is updated (`README` & prompt).
+
+## Technical Notes
+- **Integration Approach:** Use `PrefabUtility.SaveAsPrefabAsset` after creating a temporary `GameObject` with the script component.
+- **Existing Pattern Reference:** Mirror structure of `AddAssetToSceneTool.cs` & its TS wrapper.
+- **Key Constraints:** Runs in **Unity Editor** context only; requires compiled script (assembly reload complete).
+
+## Definition of Done
+- [x] Functional requirements met (1-3)
+- [x] Integration requirements verified (4-6)
+- [x] No console errors; Unity refresh clean (7-8)
+- [x] Documentation updated (9)
+- [ ] PR reviewed & merged
+
+## Minimal Risk Assessment
+| Item | Description |
+|------|-------------|
+| **Primary Risk** | Script not yet compiled when tool runs → prefab lacks component |
+| **Mitigation** | Detect missing type & retry after assembly reload or return informative error |
+| **Rollback** | Delete generated prefab asset |
+
+## Compatibility Verification Checklist
+- [x] No breaking changes to existing MCP APIs.
+- [x] Prefab path is additive; does not overwrite existing assets without confirmation.
+- [x] Performance impact negligible (< 100 ms per prefab).
+
+## Validation Checklist
+- [x] Story can be completed within one development session (~4 hrs).
+- [x] Integration approach is straightforward, mirrors existing patterns.
+- [x] Acceptance criteria are testable via manual invocation.
+- [x] Rollback is simple (delete asset).
+
+## Success Criteria
+1. ✅ Prefab is created automatically with the correct component and serialized values.
+2. ✅ Developer workflow is reduced to one MCP call.
+3. ✅ No regressions in existing MCP functionality.
+4. ✅ Clear docs allow any dev to use the tool immediately.
+
+## Important Notes
+- If additional complexity (multi-prefab generation, asset dependencies) arises, escalate to **brownfield-create-epic**.
+- Preserve existing folder structure; do not hard-code absolute paths.
+- Prioritize developer ergonomics and clear error messages.