LeanProbe 0.4.0

• Split the agent contract out of AGENTS.md into an installable skill at
  src/lean_probe/skill/SKILL.md (YAML frontmatter + the MCP tool contract).
• New lean-probe install-skill drops it into agent clients' skills
  directories — ~/.claude/skills and ~/.codex/skills share the same
  skills/<name>/SKILL.md layout. Default installs to every present client
  ("both if both"); --client claude|codex forces one, --skills-dir PATH
  targets a root, --dry-run/--print preview. The skill ships inside the
  wheel, so pip install lean-probe && lean-probe install-skill needs no
  repo checkout.
• A plain pip install lean-probe stays skill-free — the skill is purely
  opt-in and a regular install writes nothing to ~/.claude/~/.codex.
• The README links to the skill as the single source of truth; a test keeps its
  tool table in sync with TOOL_NAMES.