PoC: port `check-prerequisites` to Python with output-parity tests

[mnriem]

Part of #3277. Depends on the py script type foundation.

Why: The smallest self-contained command that exercises git lookup, path resolution, file checks, and both JSON + text output — ideal to prove the pattern.

Tasks

• Implement scripts/python/check_prerequisites.py replicating all flags: --json, --paths-only, --require-tasks, --include-tasks, --help.
• Port the needed common.sh / common.ps1 helpers (feature-path resolution, branch detection) into a shared Python module.
• Replace jq logic with stdlib json.

Acceptance

• Golden-output tests assert normalized parity between .sh, .ps1, and .py for each flag combination on a fixture repo.