fix: add missing .env.example files to agent examples (closes #35)

[aasthakhatri11]

Summary

Added missing .env.example files to 18 agent example folders that require environment variables or API keys but had no setup guidance. Each file documents the required variables with placeholder values and links to where users can obtain the actual keys.

Type of Change

• New agent example
• Bug fix
• Documentation update
• Refactor / cleanup
• Other

Checklist

• I have starred this repository.
• New community agents are under contributors/<agent-name>/ (not repo root).
• I ran ruff check ..
• I ran ruff format ..
• I added/updated README.md for changed example(s).
• I added .env.example if environment variables are required.
• I added demo image/GIF (if applicable).
• I added agent profile link (if applicable).
• I updated contributors/CHANGELOG.md for community agent changes, or root CHANGELOG.md for other non-doc changes.
• I added my agent to the Community Contributors table in root README.md (if new agent).
• I verified paths/commands used in docs.
• I understand this PR requires maintainer review before merge (review-required CI).

Related Issue

Closes #35

Notes for Reviewers

Checked each folder's Python files directly to get the exact variable names. Folders where everything was hardcoded (like frontend-integration) were skipped. The style matches the existing .env.example files already in the repo.

[gautammanak1]

No description provided.