Docs and migration/deprecation plan for Python scripts

[mnriem]

Part of #19. Depends on templates carrying py: lines.

Tasks

• Update AGENTS.md ("Argument Patterns", "Command File Formats", "Adding a New Integration" where script types are referenced).
• Update README / devcontainer notes as needed.
• Define when/whether sh / ps get deprecated once py reaches full parity and adoption.

Acceptance

• Documentation reflects the py option and the migration path.

Part of: #19

[mnriem]

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