Add Things Cloud agent skill wrapper

[pdurlej]

Summary

• Add a publishable skills/things-cloud/SKILL.md wrapper for OpenClaw, ClawHub, Codex, Claude Code, and MCP-compatible agent hosts.
• Document safe Things Cloud task workflows: MCP preferred, CLI fallback, dry-run before writes, completed/logbook evidence, recurring tasks, and credential policy.
• Add docs/integrations/openclaw-publishing.md with ClawHub publish commands and OpenClaw integration request copy.
• Link the skill from README, AGENTS.md, CLAUDE.md, and llms.txt.

Publishing Notes

• clawhub CLI is available through npm, and clawhub skill publish --help confirms the publish flags used in docs.
• Actual ClawHub publish is blocked until the maintainer runs clawhub login.
• OpenClaw integration/showcase submission should happen after this PR is merged so links point to main.

Verification

• Parsed skills/things-cloud/SKILL.md YAML frontmatter with Ruby YAML.safe_load.
• git diff --check
• ASCII scan for new/changed public docs, excluding pre-existing CLAUDE.md Unicode.
• GOCACHE=/private/tmp/go-build-things go test ./...