docs: add Phase 8C MCP design-only plan

Adds a docs-only Phase 8C MCP design plan for the CompText / Codex operating layer.

Validation:
- Rust Validation CI / validate: SUCCESS
- Gemini false-positive review thread resolved
- changed files: docs/PHASE8C_MCP_DESIGN_ONLY.md only
- PR marked ready for review before merge

Safety:
- no MCP activation
- no .mcp.json
- no .codex/** edits
- no .agent/skills/** edits
- no hooks/plugins/global skills activation
- no provider/model calls
- no source/report/artifact changes

Author: ProfRandom92