docs: add generic MCP extension framework ADR

[vishu-bh]

🔗 Related Issue

Closes #4957

---

📝 Summary

Adds ADR-053 for the generic MCP extension framework and updates the ADR index.

The ADR records the framework-first decision for MCP extensions: official extensions use capabilities.extensions, simple future extensions can be onboarded declaratively without core dispatcher changes, and MCP Apps is treated as the first complex extension built on top of the generic framework.

---

🏷️ Type of Change

• Bug fix
• Feature / Enhancement
• Documentation
• Refactor
• Chore (deps, CI, tooling)
• Other (describe below)

---

🧪 Verification

Check	Command	Status
Diff hygiene	git diff --check	Passed
Docs build	make -C docs build	Not run: docs/.venv is not present
Lint suite	make lint	Not run: docs-only ADR change
Unit tests	make test	Not run: docs-only ADR change
Coverage ≥ 80%	make coverage	Not run: docs-only ADR change

---

✅ Checklist

• Code formatted (make black isort pre-commit)
• Tests added/updated for changes
• Documentation updated (if applicable)
• No secrets or credentials committed

---

📓 Notes (optional)

This PR is the approval gate for the grouped delivery stories under #2527.