feat: refuse skills rewritten from Claude to Codex

[hibeekaey]

Summary

• Codex Desktop's import from Claude Code rewrites every "Claude" in skill text to "Codex" (claude -p became Codex -p). On one machine that damage reached ~/.claude/skills, and agent skills sync copied it to every agent: 46 files across 12 skills.
• agent skills sync now refuses a damaged skill, lists each file on stderr, still syncs the clean skills, and exits nonzero.
• agent status and agent doctor report damaged skill files as a problem.
• Patterns match only text no legitimate skill contains (Codex Code, Codex, Codex, Codex -p, Codex.ai, Codex's judgment/verdict); zero hits across every installed skill on both machines (about 7,500 files on one of them).

Validation

• make test and shellcheck bin/agent pass locally
• Ran on macOS (full suite) and Linux (skills suite); the new test was shown to fail with the check disabled.