v1.0.0

Add xmaster skill install/update/status commands

New: xmaster skill install — installs the bundled SKILL.md to all
detected agent platforms with one command:
- ~/.agents/skills/xmaster/ (universal — Codex, Gemini, others)
- ~/.claude/skills/xmaster/ (Claude Code / Claude Desktop)
- ~/.codex/skills/xmaster/ (Codex CLI / Codex Mac App)
- ~/.gemini/skills/xmaster/ (Gemini CLI)

Uses symlinks from platform-specific dirs to the universal location.
Falls back to copy on Windows or if symlink fails.

Commands:
- xmaster skill install — write + symlink to all platforms
- xmaster skill update — overwrite with latest bundled version
- xmaster skill status — show where installed and if current

The SKILL.md is embedded at compile time via include_str!, so the
skill version always matches the binary version.