chore(ci): add scheduled trivy scan with Linear ticket creation

[fyzanshaik-atlan]

Summary

Adds a scheduled security scan for the PyAtlan Docker image and uv.lock dependencies, with automatic Linear ticket creation when vulnerabilities are found.

What it does

• Schedule: Twice a week — Monday and Thursday at 09:00 UTC (also supports manual trigger via workflow_dispatch)
• Image scan: Builds the Docker image and scans with Trivy for HIGH/CRITICAL vulnerabilities
• Dependency scan: Scans uv.lock for known vulnerable packages
• Linear integration: Automatically creates a Linear ticket with properly formatted markdown tables showing severity, package, installed/fixed versions, and CVE links
• Workflow summary: Publishes full Trivy table output to the GitHub Actions step summary
• Clean scans: No ticket created if 0 vulnerabilities (no noise)

Approach

Self-contained inline workflow (matches the approach used in atlanhq/application-sdk). No dependency on external reusable workflows — everything runs within this repo's workflow file.

Linear ticket format

When vulnerabilities are found, the ticket includes:

• Summary table with image/dependency/total counts
• Per-section markdown tables (Docker Image / Dependencies) with severity, package, installed version, fixed version, and CVE
• Links to the repository and workflow run

Setup required

1. LINEAR_API_KEY — repository or org secret
2. LINEAR_TEAM_ID — repository secret
3. CHAINGUARD_USERNAME / CHAINGUARD_PASSWORD — already configured for this repo

[Aryamanz29]

LGTM - thanks! @fyzanshaik-atlan 🚀