feat: add LinkedIn mentions support to Typefully CLI and skill

[frankdilo]

Summary

• add a new CLI command: - linkedin:organizations:resolve [social_set_id] --organization-url <url>
  • supports --organization_url and --url aliases
• wire the command to Typefully API endpoint: - GET /v2/social-sets/{social_set_id}/linkedin/organizations/resolve
• update CLI help/examples with LinkedIn mention workflow and mention syntax
• update skills/typefully/SKILL.md with LinkedIn mentions section, command reference, and examples
• bump SKILL frontmatter last-updated to 2026-02-26
• add changelog entries in skills/typefully/CHANGELOG.md
• add tests for success/default-social-set/error cases in tests/social-sets.test.js

Validation

• npm run test:social-sets
• npm run test:all