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>

common.md

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+# Common Agent Skills
+
+## Code Style
+
+- Write clean, minimal code. No unnecessary comments unless the *why* is non-obvious.
+- Prefer editing existing files over creating new ones.
+- Do not add error handling or validation for scenarios that cannot happen.
+- Three similar lines is better than a premature abstraction.
+- No half-finished implementations.
+
+## Task Approach
+
+- For exploratory questions, respond with a recommendation and the main tradeoff in 2-3 sentences before implementing.
+- Do not add features, refactor, or introduce abstractions beyond what the task requires.
+- Default to the simplest solution that satisfies the requirement.
+
+## Security
+
+- Never introduce command injection, XSS, SQL injection, or other OWASP top 10 vulnerabilities.
+- Only validate at system boundaries (user input, external APIs). Trust internal code and framework guarantees.
+- Do not hardcode secrets or credentials in code.
+
+## Git
+
+- Write concise commit messages focused on *why*, not *what*.
+- Prefer creating new commits over amending published ones.
+- Never force-push to main/master without explicit confirmation.
+
+## Communication
+
+- Be concise. Match response length to task complexity.
+- State results and decisions directly.
+- When referencing code, include file path and line number when possible.