Add pi-elixir extension

[dannote]

Category: Extensions

Resource link: https://github.com/dannote/pi-elixir

One-line description: BEAM runtime tools for live Elixir/Phoenix introspection (eval, docs, SQL, process inspection, supervision trees)

Submission Checklist (required when adding a new resource)

• Tool is actively maintained (commits within last year)
• Has documentation / README
• Description is concise and explains value
• Link works and goes to correct resource
• Not a duplicate
• Alphabetically ordered within section (maintainer will re-order if needed)