feat: reshape Codex skills as a first-class product surface

[jonnii]

Codex skills (~/.codex/skills/) shipped today as Claude templates with
$ARGUMENTS replaced — keeping model:, AskUserQuestion, !... macros, and
~/.claude/skills paths Codex either ignores or treats as noise.

Reshape Codex output without touching Claude (which stays byte-identical):

• Add codexDescriptions corpus with trigger-phrase routing descriptions
  for all 17 commands plus the stackit router.
• Extend renderCodexSkillContent to strip model:, filter Claude-only entries
  from allowed-tools, rewrite ~/.claude paths, strip !... macros, rewrite
  cross-skill 'Skill tool' prose to direct instructions, and rewrite
  AskUserQuestion prose to plain 'ask the user'.
• Apply the same frontmatter + body transforms to file-group installs so
  router SKILL.md, references/, and subagent files all get the treatment.
• Add a templates/codex_overrides/ tree with per-file precedence: slim
  router SKILL.md, three references (workflows, commit-style,
  stack-plan-recovery), and execution-checklist bodies for stack-create
  and stack-plan.
• Wire format through installFileGroup and installCommandSkills so the
  override loader is keyed off agentSkillFormatCodex.
• Add tests: override precedence, full-tree banned-substring lint, Claude
  byte-identical guard, and per-transform unit cases.