feat(agent): replace contributor GEMINI.md with user-facing AGENTS.md and add rules/sre-extension.md #57

Problem

1. Currently, root GEMINI.md contains contributor git branching instructions instead of the SRE agent persona (which sits unused in docs/sre-agent-personality.md).
2. There is no AGENTS.md (and symlinks are brittle on Windows).
3. There is no rules/ directory for Antigravity plugin skill auto-discovery.

Proposed Fix

1. Move contributor git/pre-commit rules from GEMINI.md into CONTRIBUTING.md, leaving a 1-line reference in AGENTS.md.
2. Create a single cross-platform AGENTS.md at the root (incorporating docs/sre-agent-personality.md + skill routing table) and set contextFileName to AGENTS.md in gemini-extension.json, removing GEMINI.md (no symlinks).
3. Add rules/sre-extension.md so Antigravity automatically routes SRE/GCP tasks to the extension skills.