Add .env.example templates + update sample READMEs to use safe secret setup

[goolig]

Title

Add .env.example templates + update sample READMEs to use safe secret setup

Summary

Several samples in this repo instruct users to create a .env file containing API keys (e.g., GEMINI_API_KEY) but the repo does not provide any .env.example / template files. This encourages insecure “copy/paste” workflows and increases the risk of accidentally committing real secrets.

Problem

Today, multiple sample READMEs tell developers to do things like:

• echo "GEMINI_API_KEY=..." > .env
• export GEMINI_API_KEY=... and/or append to .env

…but there is no corresponding template file (.env.example, .env.local.example, etc.) that:

• documents required env vars,
• distinguishes placeholders vs real secrets,
• standardizes setup with cp .env.example .env.

This is especially risky in a reference repo because users tend to treat it as “best practice”.

Why this matters (security + developer experience)

• Reduces accidental secret leakage (commits, screenshots, copy/paste into tickets)
• Sets a strong precedent for credential hygiene in sample code
• Makes onboarding faster and more consistent

Proposed change

1. Add .env.example (or .env.local.example where appropriate) to each sample that requires secrets.

   • Include placeholders + brief comments.
   • Example:

   # Copy to .env and fill values
   GEMINI_API_KEY=your_gemini_api_key_here
   
   # Optional:
   # GOOGLE_MAPS_API_KEY=your_google_maps_api_key_here

2. Update sample READMEs to use the standard pattern:

   cp .env.example .env
   # edit .env (do not commit)

3. Ensure .env files are ignored repo-wide:

   • Add to root .gitignore if missing:

     • .env
     • .env.* (optional, depending on conventions)
     • !.env.example (ensure templates are kept)

Acceptance criteria

• Every sample that needs an API key has a matching .env.example (or .env.local.example) committed
• All READMEs stop recommending echo "...=REAL_KEY" > .env and instead recommend copying from the template
• .env is ignored by git while templates remain tracked
• Quickstart instructions remain copy/paste friendly

Suggested locations (examples)

• samples/agent/... (ADK sample(s) that require GEMINI_API_KEY)
• samples/client/... (Angular / other client samples)
• Any other sample folders referencing *_API_KEY, api_key, GEMINI_API_KEY, etc.

This is standard practice in more mature projects—for example, Vercel’s official Next.js + Supabase example ships a .env.local.example template, and projects like Supabase and n8n include .env.example files as the default onboarding flow for secrets.