docs(project): complete installation and usage guide

[batuthzcode]

Summary

• publish cross-platform installation instructions for Windows, Linux, and macOS
• document static analyzer text, JSON, exit-code, and severity-gate usage
• add --fail-on any|info|warning|error support to the static analyzer
• document live OSV queries and deterministic offline dependency scans
• describe dependency scanner parameters, output formats, and failure policy
• document project limitations and security boundaries
• add contract tests for the public README
• update the documented test count and coverage result

Verification

• 1065 tests passed
• 98.6% combined statement/branch coverage (97% required)
• sample-app reports are current
• whole-project self-analysis report is current
• TOML and GitHub Actions YAML parse successfully
• Python compileall completed successfully
• --fail-on error returned 0 for the controlled demo
• --fail-on warning returned 1 for the controlled demo
• git diff --check passed

Backlog

Completes Backlog 5.6, including tasks 5.6.1 through 5.6.4.