feat: support Trae IDE

[WilliamMunch]

Description

Integrate the Trae AI agent into the Spec Kit. It provides full support for CLI, context update scripts (Bash/PowerShell), release automation, and documentation.

Changes

1. init.py: Added trae to AGENT_CONFIG as a supported IDE-based agent (configured to use .trae/ folder).
2. update-agent-context.sh、update-agent-context.ps1: Added logic to handle the trae argument and update context files in .trae/rules/specify-rules.md.
3. create-release-packages.sh、create-release-packages.ps1: Added generation logic for Trae slash command files.
4. create-github-release.sh: Configured to include Trae artifacts (spec-kit-template-trae-*.zip) in official GitHub releases.
5. AGENTS.md: Documented Trae as a supported agent, including its directory convention (.trae/commands/).
6. README.md: Added Trae to the list of supported AI agents.