Port core scripts to Python (common, create-new-feature, setup-plan, setup-tasks)

[mnriem]

Part of #19. Depends on the check-prerequisites PoC.

Tasks

• Port common, create-new-feature, setup-plan, setup-tasks to Python.
• Reuse the shared helper module established by the PoC.
• Add parity tests per script (text + JSON output, all flags).

Acceptance

• Normalized output parity between .sh, .ps1, and .py for each script.

Part of: #19

[mnriem]

Recreated in upstream github/spec-kit (github#3277 and sub-issues). Closing fork duplicate.