feat(agent): Implement Plan Mode - Plan Phase with capability separation

[ChaiWithJai]

Summary

Implement Plan Phase of the Plan/Act/Verify framework for Violet App Agent.

Details

Based on RFC-001, implement the Plan Phase that:

1. Acknowledges user vision immediately with streaming feedback
2. Separates CAN_BUILD from YOULL_ADD capabilities
3. Shows YAML specification preview
4. Allows user to Approve / Modify / Start Over

Subagents Involved

• Architect Subagent: Data model and API design
• Security Subagent: Review for vulnerabilities

Artifacts Produced

• App specification (YAML)
• Capability breakdown (CAN_BUILD vs YOULL_ADD)
• Estimated build steps

Acceptance Criteria

• Streaming UI shows 'Understanding your vision...' immediately
• Capability breakdown shows what agent can vs can't do
• YAML specification is displayed for user review
• User can approve, modify, or start over

Reference

• RFC-001: docs/RFC-001-plan-mode-ux.md
• Related PR: feat(agent): Web UI Agent for natural language app deployment #1719