feat: add Reasonix agent skill adapter

Add a new agent adapter for the Reasonix coding agent, following the
same pattern as the existing Codex and Hermes skill adapters.

New files:
- reasonix-skill/SKILL.md — full CLI-Anything methodology mapped to
  Reasonix's tool ecosystem (bash, write_file, edit_file, multi_edit,
  grep, glob, mcp__codegraph__*, task, web_fetch)
- reasonix-skill/agents/reasonix.yaml — agent interface metadata
- reasonix-skill/scripts/install.sh — macOS/Linux installer
- reasonix-skill/scripts/install.ps1 — Windows PowerShell installer
- reasonix-skill/README.md — installation and usage documentation
- reasonix-skill/REASONIX_SKILL_INSTALL.md — install_skill API reference

Modified files:
- .gitignore — allow reasonix-skill/ directory
- README.md — add Reasonix to supported agent list, project structure,
  quick-start prerequisites, and Works with line