chore: simplify pipx usage and remove redundant example

[clouatre]

Summary

Simplifies linting commands by using pre-installed pipx directly, and removes redundant Security Scan example.

Changes

Simplified pipx usage (3 locations)

Before:

run: |
  pipx install uv
  uv tool run ruff check --output-format=json . > lint.json || true

After:

run: pipx run ruff check --output-format=json . > lint.json || true

Updated in:

• .github/workflows/test-tier1.yml
• README.md (Quick Start)
• examples/tier1-maximum-security.yml

Removed redundant example

• Removed "Security Scan with Artifact Upload" example from README
• Same pattern as Quick Start, just different AI provider
• Reduces documentation duplication

Why

• Faster: No intermediate uv installation needed
• Simpler: Single line instead of 3
• Pre-installed: Uses pipx 1.8.0 bundled with Ubuntu 24.04 runners
• Cleaner docs: Removes redundant example that doesn't add educational value

Testing

• ✅ Verified pipx 1.8.0 is pre-installed on Ubuntu 24.04 runners
• ✅ Tested pipx run ruff works locally
• ✅ No functional changes, only simplification