feat(install-mcp): `openclaw` host adapter

[plind-junior]

What you're trying to do

The vouch install-mcp <host> story (#180) covers eight hosts. OpenClaw isn't one of them — a contributor running vouch install-mcp openclaw from inside a project today fails. Add a manifest-driven adapter so OpenClaw users get the same auto-wiring as Claude Code users: drop a .openclaw/plugin.toml (or wherever OpenClaw expects per-project plugin enables), fenced snippet into AGENTS.md, settings hook.

Suggested shape

• New adapters/openclaw/:
  • install.yaml declares T1 (openclaw plugin enable vouch), T2 (fenced AGENTS.md snippet pointing at the manifest), T3 (slash commands — symlinks to existing under adapters/claude-code/.claude/commands/), T4 (per-project policy file).
• Template snippet teaches the OpenClaw host about the review gate.
• tests/test_install_adapter.py parametrised over hosts picks up openclaw automatically.

Acceptance

• vouch install-mcp openclaw --path /some/project writes the four tiers idempotently.
• The CodeRabbit / manifest-validation gate (feat(cli): vouch install-mcp <host> — one-command adapter writer (8 hosts) #180) catches malformed install.yaml.

Out of scope

• Auto-detecting an OpenClaw deployment without an explicit flag.