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

I've manually verified my changes. Read my comment below.

Visual Changes

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Reviewer Notes

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[kilo-code-bot]

Code Review Summary

Status: 1 Issue Found | Recommendation: Address before merge

Executive Summary

CALLBACK_TOKEN_SECRET=changeme introduces the same predictable placeholder pattern that was just fixed for BYOK_ENCRYPTION_KEY; both are cryptographic secrets that should be empty (forcing explicit setup) rather than set to a known value.

Overview

Severity	Count
CRITICAL	0
WARNING	1
SUGGESTION	0

Issue Details (click to expand)

WARNING

File	Line	Issue
.env.local.example	17	CALLBACK_TOKEN_SECRET=changeme — predictable placeholder for a cryptographic signing secret; prefer empty value (matches the pattern used for BYOK_ENCRYPTION_KEY immediately below)

Resolved Issues

• .env.local.example line 19 — BYOK_ENCRYPTION_KEY=changeme ✅ Fixed: now set to empty value with generation instructions
• ENVIRONMENT.md duplicate entries — ✅ File is now a fresh addition with no duplicates

Files Reviewed (3 files)

• .env.local.example — 1 issue
• CONTRIBUTING.md — no issues
• ENVIRONMENT.md — no issues

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Fix these issues in Kilo Cloud

_{Reviewed by claude-sonnet-4.6 · incremental review}

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

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_{Reviewed by claude-4.6-sonnet-20260217 · 414,085 tokens}

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

[IamCoder18]

I've verified most of my changes by following all given instructions while working on another PR except the seeding-related instructions. The ENVIRONMENT.md remains only verified by AI.

Additionally, while testing, I noticed that there are two more required environment variables (CALLBACK_TOKEN_SECRET and BYOK_ENCRYPTION_KEY) for the dev server to work. I've modified the example env file to move these two variable up. I've also modified the CONTRIBUTING.md to include these and to mention the Stripe keys required for authentication.

This PR is now read for review.