feat(codex): add global CRG skill with safe CLI fallback

[CissiBot]

Summary

• install a global Codex code-review-graph Skill from the package
• prefer CRG MCP tools and fall back to a bundled read-only CLI wrapper
• honor the active CODEX_HOME for Codex MCP, hooks, Skill install, and uninstall
• protect existing user Skill files and avoid implicit graph build/update when a graph is missing, empty, or stale

Closes #841

Why

Codex runtimes in WSL and Windows have separate CODEX_HOME directories and tool inventories. A project MCP entry can therefore be present in one runtime while the Codex session executing the task cannot discover it. A global Skill gives Codex durable instructions and a local fallback while preserving the existing MCP integration.

Safety

• the Skill performs one read-only health check per repository/task
• graph mutations (build, update, postprocess, embed, watch) require explicit user intent
• the CLI fallback exposes read-only commands only and resolves the repository explicitly
• installation is idempotent, manifest-backed, symlink/path-traversal safe, and preserves unmarked or user-edited files
• uninstall removes only CRG-owned files that still match their recorded hashes

Verification

• 254 passed (targeted install, Skill, uninstall, and documentation tests)
• Skill validation passed
• Ruff passed on changed Python files
• compileall passed
• git diff --check passed
• wheel and sdist both include the three Skill resources