Add Sifa Professional Network lexicons (id.sifa.*)

[gxjansen]

Adds Sifa to the list. It's a professional identity and career network on AT Protocol where profiles, skills, endorsements, and follow relationships all live in the user's PDS.

• https://sifa.id
• Namespace: id.sifa.*
• Lexicon repo: https://github.com/singi-labs/sifa-lexicons
• Bluesky: @sifa.id
• License: MIT

Lexicons

Lexicon ID	Description
id.sifa.profile.self	Professional profile summary (headline, about, industry, location)
id.sifa.profile.position	Work history (company, title, dates)
id.sifa.profile.education	Education history
id.sifa.profile.skill	Professional skills
id.sifa.profile.certification	Professional certifications
id.sifa.profile.project	Portfolio projects
id.sifa.profile.volunteering	Volunteer experience
id.sifa.profile.publication	Publications
id.sifa.profile.course	Courses
id.sifa.profile.honor	Honors and awards
id.sifa.profile.language	Language proficiencies
id.sifa.endorsement	Skill endorsement (from another user)
id.sifa.endorsement.confirmation	Endorsement confirmation (mutual consent)
id.sifa.graph.follow	Professional follow relationship