Add pytest-cov coverage gate

[askmy-stack]

Background

Identified during OSS audit v2 of cortex.

Problem Statement

No coverage threshold.

Proposed Solution

70% on api/ and graph/.

Acceptance Criteria

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

Files / Modules Affected

.github/workflows/ci.yml

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Suggested labels: priority:medium, difficulty:beginner, category:testing
Milestone: v2.0-features
OSS Audit: Master Backlog v2