[Agent] Add Trae.ai IDE support

Type: New agent integration

Description: Trae.ai is an AI IDE (similar to Cursor/Windsurf) with growing adoption. It supports project-scoped rule files at .trae/rules/*.md, which is the same mechanism used by Windsurf (.windsurfrules) and Cline (.clinerules).

Proposed integration:

• Method: rules file (.trae/rules/rtk-rules.md) — same as Windsurf/Cline fallback
• Command: rtk init --agent trae
• No preToolUse hook available (Trae doesn't expose shell interception), so RTK would rely on the AI following the rules to call rtk explicitly

References:

• Trae rules docs: https://docs.trae.ai/ide/rules
• Similar existing integrations: Windsurf, Cline, Kilo Code