Initial agent skills setup

Common skills shared across Claude Code, Codex, and Antigravity,
plus per-agent extras. sync.sh deploys to each tool's config path.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

agents_extra.md

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+# Codex & Antigravity — Agent-Specific Skills
+
+## Skill Discovery
+
+- Skills in `~/.agents/skills/[name]/SKILL.md` are auto-discovered.
+- Each skill directory can include optional `scripts/`, `references/`, and `assets/` subdirectories.
+- Skills can be scoped: user (`~/.agents/skills`), repo (`.agents/skills`), or system (`/etc/codex/skills`).
+
+## SKILL.md Format
+
+- Always include YAML frontmatter with `name` and `description` fields.
+- `description` should clearly state when and how the skill applies.
+- Example:
+  ```yaml
+  ---
+  name: my-skills
+  description: Personal coding conventions and preferences
+  ---
+  ```
+
+## Task Execution
+
+- Prefer running tasks in isolated environments when available.
+- Use `agents/openai.yaml` to configure agent-specific behavior per skill directory.
+- Leverage built-in Codex skills before writing custom ones.
+
+## Project Awareness
+
+- Read `.agents/skills` at repo root for project-specific overrides.
+- Project-level skills take precedence over user-level skills.