docs: Add CONTRIBUTING.md, .env.example, housekeeping

[dean0x]

Problem

The project lacks contributor documentation, making it hard for new contributors to get started. Several housekeeping items need attention before public visibility.

Scope

1. Create CONTRIBUTING.md

• Dev environment setup instructions
• Explain the npm test safeguard and why it exists
• Document all test groups and when to use each
• PR process and expectations
• Code style guide (reference Biome config)
• Architecture overview for new contributors

2. Create .env.example

Document all 21+ environment variables from src/implementations/configuration.ts:

• Required vs optional
• Default values
• Description of each

3. Fix .gitignore

• Remove CLAUDE.md entry (this file is checked into the repo and should be tracked)
• Deduplicate coverage/ entries
• Deduplicate .memory/ entries

4. Update LICENSE

• Update copyright year range to 2024-2026

Acceptance Criteria

• CONTRIBUTING.md exists with complete dev setup guide
• .env.example documents all environment variables
• .gitignore is clean (no CLAUDE.md exclusion, no duplicates)
• LICENSE copyright is current
• A new contributor can set up the project using only README + CONTRIBUTING

Dependencies

• Depends on all other issues (content references final state of tooling, CI, and test setup)