Add CONTRIBUTING.md with local dev guide

[askmy-stack]

Background

Identified during OSS audit v2 of cortex.

Problem Statement

No contributor onboarding.

Proposed Solution

Document make demo, env vars, tests.

Acceptance Criteria

• Implementation complete
• Tests pass
• Documented in README or docs/

Files / Modules Affected

CONTRIBUTING.md

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Suggested labels: priority:high, good first issue, category:documentation
Milestone: v1.0-oss-ready
OSS Audit: Master Backlog v2