Add gitagent format support (agent.yaml, SOUL.md, RULES.md, DUTIES.md)

[shreyas-lyzr]

Summary

• Adds gitagent standard metadata files so gstack is also a valid gitagent agent
• Fully additive — zero changes to existing gstack files, everything continues to work exactly as before
• All 27 skills mapped to gitagent-compatible skills/ directory

What is gitagent?

gitagent is a framework-agnostic, git-native standard for defining AI coding agents. It lets agent configs be exported to any platform — Claude Code, Cursor, Copilot, OpenAI, Codex, Gemini CLI, and more — from a single source of truth.

gstack is a perfect fit: it's already Markdown-first, skill-based, and git-native. This PR makes that compatibility explicit.

New files

File	What it does
agent.yaml	gitagent manifest — name, version, 27 skills, model preference, tags
SOUL.md	Builder philosophy extracted from ETHOS.md (Boil the Lake, Search Before Building, Three Layers of Knowledge)
RULES.md	Development rules extracted from CLAUDE.md (platform-agnostic design, commit style, testing requirements)
DUTIES.md	Sprint workflow — Think → Plan → Build → Review → Test → Ship → Reflect, with each skill's role
hooks/hooks.yaml	Safety hooks (freeze check, careful mode)
skills/*/SKILL.md	gitagent-compatible wrappers for all 27 skills (same instructions, schema-compatible frontmatter)

Validation

$ gitagent validate
✓ agent.yaml — valid
✓ SOUL.md — valid
✓ hooks/hooks.yaml — valid
✓ skills/ — valid (21 warnings — token length, expected for detailed skills)
✓ Validation passed

What this enables

With gitagent format, gstack can be:

• Exported to any adapter: gitagent export -f cursor, gitagent export -f copilot, gitagent export -f opencode, etc.
• Discovered and installed from the gitagent registry
• Validated with gitagent validate
• Compared across versions with gitagent diff

Test plan

• gitagent validate passes cleanly
• gitagent info shows all 27 skills
• Existing gstack ./setup and skill invocation unchanged
• No existing files modified