feat(cli): complete ScanCode-facing CLI rollout

[mstykow]

Summary

• complete the ScanCode-facing CLI rollout for package scan modes, --info gating, --license-policy, --max-in-memory, compiled package extraction, and Debian output parity
• add focused parity coverage for Debian output and tighten the generated CLI/help/docs surfaces around required flag combinations and first-time-user workflows
• add a dedicated CLI guide while keeping README.md aligned with --help as the exhaustive flag reference

Issues

• Covers: docs/implementation-plans/infrastructure/CLI_PLAN.md
• Closes:

Scope and exclusions

• Included:
  • CLI/runtime parity work for the remaining ScanCode-facing options on the rollout branch
  • Debian output parity fixes, fixture-backed verification, and output parity scorecard coverage
  • first-time-user CLI documentation and README/docs index cleanup
• Explicit exclusions:
  • unrelated parser or non-CLI feature work outside the rollout scope

Intentional differences from Python

• None beyond already-documented Provenant-specific behavior that predates this branch

Follow-up work

• Created or intentionally deferred:
  • none

Expected-output fixture changes

• Files changed: testdata/output-formats/debian-basic-expected.copyright
• Why the new expected output is correct:
  • it matches the current Debian writer behavior, including fixture-backed validation of DEP-5 structure and the use of detected non-SPDX license expressions before fallback file expressions