Add scan CLI for PII, adapt secret scanner API, and update packaging + tests

[Jeffrin-dev]

Motivation

• Provide a quick CLI entrypoint to scan input text for PII entities and emit JSON results for automation.
• Update secret-detection integration to match the newer detect-secrets API and improve entropy-based heuristics.
• Improve packaging metadata and optional dependency groups for NER/siem use-cases and ensure policy files are included in distributions.

Description

• Add a scan subcommand to shadowaudit.cli that constructs PIIScanner(fast_mode=True), scans input text, and prints a JSON payload of the result.
• Import asdict and PIIScanner in the CLI and fallback to model_dump() when available for pydantic/attrs-compatible output.
• Replace old detect-secrets usage in shadowaudit/core/secrets.py with the detect_secrets.core.scan API and adapt the scanning loop to extract secret_value; keep entropy-based candidate detection and merging logic.
• Update pyproject.toml to use hatchling as the build backend and add [project.optional-dependencies] groups ner and siem.
• Add MANIFEST.in to include policies/gdpr.yaml, policies/hipaa.yaml, and policies/pci_dss.yaml.
• Add a new test tests/test_cli_scan.py exercising the shadowaudit scan command and asserting that an EMAIL entity is reported.

Testing

• Ran pytest tests/test_cli_scan.py which executed the new CLI test and the test passed.
• Project test run (via pytest) completed for the modified test target without failures.

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Codex Task