Fix OpenClaw pluginApi compatibility drift in capabilities metadata

[Copilot]

The failing ci / test (py3.13) job was caused by drift between the OpenClaw plugin manifest and the kb.capabilities response. The manifest was missing openclaw.compat.pluginApi, and capabilities did not advertise the corresponding host_compat.openclaw.pluginApi entry expected by the compatibility checks.

• OpenClaw compatibility metadata

  • add openclaw.compat.pluginApi to openclaw.plugin.json
  • keep the declared plugin API floor explicit in the manifest rather than only in package.json

• Capabilities surface

  • extend Capabilities with host_compat
  • expose host_compat.openclaw.pluginApi from src/vouch/capabilities.py

• Drift checks

  • add coverage in tests/test_capabilities.py for the new host_compat contract
  • update tests/test_openclaw_plugin_manifest.py to assert manifest/package compatibility parity
  • stop treating openclaw as a dead manifest field now that it is part of the current contract

• Schema/docs alignment

  • add host_compat to schemas/capabilities.schema.json
  • update CLAUDE.md to document that the manifest also carries the OpenClaw compatibility floor

{
  "openclaw": {
    "compat": {
      "pluginApi": ">=2026.6.0"
    }
  }
}

host_compat={"openclaw": {"pluginApi": ">=2026.6.0"}}