[4.1] Enrich CLAUDE.md template content

[vbonk]

What needs to be done

Expand from 67 lines to ~150 lines. Add: architecture overview, key decisions log, env vars table, testing strategy, deployment, error handling, dependencies, prompt injection awareness section, see-also footer. Cite Anthropic best practices.

Acceptance Criteria

• ~150 lines of rich template content
• Prompt injection awareness section
• All sections use TODO markers
• Cross-linked to AGENTS.md, copilot, AI-SECURITY.md

[vbonk]

Completed in Phase 4+5 PR #88.