Add SECURITY.md for API keys and data handling

[askmy-stack]

Background

Identified during OSS audit v2 of startupintel.

Problem Statement

No security policy.

Proposed Solution

SECURITY.md.

Acceptance Criteria

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

Files / Modules Affected

SECURITY.md

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