feat: add Agent First skill and update READMEs (#80)

* feat: add Agent First skill and update READMEs

- Add skills/SKILL.md with agent-oriented Skill definition (scope, capabilities, typical queries, quick start)
- Add skills/references/firstdata-register.md with token registration, challenge verification, and MCP configuration
- Update README.md/README.en.md/README.ja.md: add Agent First branding, Skill badge, Agent Skill row in core advantages table, dual-path Quick Start (Agent auto-onboarding + manual MCP config), consolidate platform configs into single universal JSON + reference table

* fix(skill): remove openclaw nesting from metadata

Flatten metadata fields by removing the unnecessary openclaw hierarchy level.

* refactor(skill): move skill files into skills/firstdata/ subdirectory

* fix(skill): align frontmatter with ClawHub skill-format spec

- Restore metadata.openclaw nesting per spec requirement
- Add requires.env declaration for FIRSTDATA_API_KEY
- Add emoji field
- Rewrite Quick Start with explicit MCP config and transparent service info

* chore(skill): bump version to 0.0.2 and format table alignment

* docs: add ClawHub install command and update skill paths in READMEs

- Add ClawHub badge linking to clawhub.ai/ningzimu/firstdata
- Replace verbose agent onboarding steps with `clawhub install` command
- Update skill file paths from skills/SKILL.md to skills/firstdata/SKILL.md

* fix(docs): correct clawhub install slug to firstdata

Author: ningzimu