docs: improve contributing and environment documentation

[IamCoder18]

Summary

Added two documentation files to lower the barrier for community contributors:

• CONTRIBUTING.md — a practical onboarding guide covering prerequisites, local development setup, database seeding with pnpm dev:seed, and common development commands. Addresses the gap where the previous guide was macOS-focused and lacked concrete local testing tooling references.
• ENVIRONMENT.md — a comprehensive reference of all environment variables used across the monorepo, organized by domain (App, KiloClaw, Cloud Agent, etc.) with descriptions of what each variable activates. Provides the transparency needed for non-employees to understand which keys are required for specific product surfaces without access to vercel env pull.

These changes directly address the onboarding and environment documentation gaps raised in #3510, making it easier for external contributors to set up their environment and understand what each configuration variable does.

Closes #3510

Verification

N/A

Visual Changes

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Reviewer Notes

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[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Executive Summary

All 5 duplicate variable entry warnings from the previous review have been resolved; ENVIRONMENT.md now documents each variable exactly once.

Files Reviewed (2 files)

• CONTRIBUTING.md — no issues
• ENVIRONMENT.md — all previous duplicate entries removed

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_{Reviewed by claude-4.6-sonnet-20260217 · 689,179 tokens}

_{Review guidance: REVIEW.md from base branch main}