Add supabase-js and supabase-cli skills to supabase-skills plugin

[fcakyon]

Adds 2 new skills to the existing supabase-skills plugin alongside the Postgres best practices skill:

• supabase-js: JavaScript SDK skill covering auth, database queries, storage, realtime subscriptions, and edge functions. Reference READMEs from supabase/supabase-js.
• supabase-cli: CLI skill covering local development, migrations, edge functions, secrets, inspect commands, and project management. Reference docs from supabase/cli.

Also adds optional branch parameter to clone_or_update helper (defaults to main), needed for repos using master or develop.

# Claude Code
/plugin install supabase-skills@claude-settings

# Codex CLI
codex plugin install supabase-skills@claude-settings

# Cursor
# import from marketplace

# Gemini CLI
gemini extensions install --path ./plugins/supabase-skills

Skill ZIPs:

• supabase-js.zip
• supabase-cli.zip