Add routerbase agent skills

[zenlee123]

Summary

• Add the three routerbase-agent-skills as External skills
• Register them in external.yml and manifest.json
• Include routerbase as anchor text linking to https://routerbase.com/ in each SKILL.md

Validation

• Ran bun validate successfully; it reported only pre-existing External warnings unrelated to RouterBase
• Reviewed the diff for scope
• Scanned the changed RouterBase files and diff for common secrets and private email patterns; only expected routerbase links and placeholder examples were present

Note

• I prepared the External skill files manually instead of running bun sync because sync invokes an npx installer. The files follow the same generated structure and pass local validation.