feat(prompts): Prompt Engineering Toolkit — 15 curated templates + API

- Add PromptTemplates.swift: 15 curated templates across 9 categories
  (Review, Debug, Generate, Refactor, Test, Document, Security, Performance, Deploy)
  with variable substitution, tags, and Hashable conformance
- Add PromptTemplatesView.swift: SwiftUI browser with category filter,
  search, variable form, live preview, Send to Chat / Load into Input actions
- Add GET /api/prompts — list all templates
- Add GET /api/prompts/{id} — template with variables
- Add POST /api/prompts/render — render template with filled variables
- Bump version to 6.4.0 build 9

Templates: Swift Code Review, Bug Fix, Feature Implementation, Refactor File,
Add Unit Tests, Add Documentation, Security Audit, Performance Analysis,
Explain Code, Add API Endpoint, Migration Plan, Review PR/Diff, Fix Build
Error, Add Error Handling, Create Data Model

Written by Jordan Koch.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: kochj23