Add missing env example files for example apps

[vishxrad]

Summary

Several example READMEs tell users to copy an environment template, but the referenced template file is missing. This creates unnecessary setup friction for first-time users.

Missing files referenced by docs

• examples/vercel-ai-chat/.env.example is referenced from examples/vercel-ai-chat/README.md
• examples/multi-agent-chat/.env.example is referenced from examples/multi-agent-chat/README.md
• examples/supabase-chat/.env.local.example is referenced from examples/supabase-chat/README.md

Suggested implementation

Add the missing env template files with placeholder values matching the variables used by each example. For example:

OPENAI_API_KEY=sk-...

For Supabase, include the public Supabase variables documented in the README.

Acceptance criteria

• The README copy commands work as written.
• Each new template contains placeholder values only, never real secrets.
• The variable names match what the app code reads.