⚙️ FEATURE-#22: Refactor template execution contract around dotflow start --workflow factory

[FernandoCelmer]

Summary

Unify every cloud platform template around a single execution contract built on dotflow start --workflow <module>.workflow:main. Fixes silent bugs in execution_mode (was dead config), double scheduling when scheduler=cron combined with externally-scheduled clouds, and mixed-stale entrypoints across Dockerfiles/handlers. Adds a new dotflow-cloud platform (README-only), wires EXECUTION_MODE through the post-gen hook, and bumps the generated project's dotflow pin to 1.0.0.dev2.

Changes by layer

Cookiecutter / hook:

• cookiecutter.json — add dotflow-cloud to cloud choices.
• hooks/post_gen_project.py — expose EXECUTION_MODE placeholder.

Generated project ({{cookiecutter.project_name}}/):

• pyproject.toml — bump pin to 1.0.0.dev2; Jinja override forces scheduler="none" for clouds that schedule externally.
• {{cookiecutter.module_name}}/workflow.py — main() is now a factory; adds internal workflow.start(mode=..., resume=...) only for Lambda/alibaba clouds that need it; SchedulerCron + [scheduler] extra dropped for externally-scheduled clouds.
• README.md — local run instruction updated to dotflow start --workflow ...:main --mode ....

Container-style clouds (13 platforms):

• Dockerfile/Procfile/heroku.yml/fly.toml/railway.json/github-actions dotflow.yml all use dotflow start --workflow {{MODULE_NAME}}.workflow:main --mode {{EXECUTION_MODE}}.

Lambda-compatible clouds (7 platforms):

• handler.py simplified to main() (start is internal to main() via Jinja).
• Dockerfile CMD stays on platform contract (handler.handler for Lambda, python -c ENTRYPOINT for alibaba-fc).

New dotflow-cloud platform:

• Added cloud/dotflow-cloud/README.md + registry entry with files: ["README.md"].

Docs (14 READMEs):

• "Important" section updated across all platforms to describe the new contract.

Scheduled-cloud override

workflow.py and pyproject.toml both use:

{%- set scheduled_clouds = ["dotflow-cloud", "lambda-scheduled", "ecs-scheduled", "cloud-run-scheduled", "alibaba-fc-scheduled", "github-actions"] -%}
{%- set scheduler = "none" if cookiecutter.cloud in scheduled_clouds else cookiecutter.scheduler -%}

Prevents double-scheduling when the user picks scheduler=cron together with an externally-scheduled cloud.

Known gap (tracked upstream)

checkpoint=yes combined with a CLI-based cloud still silently does nothing because dotflow start CLI is missing a --resume flag. Tracked in dotflow-io/dotflow#267. Once shipped, container-style Dockerfiles can emit --resume conditionally.

Test plan

• Manual grep: no python -m ... workflow left, no result = main() left, single CMD shape per cloud family.
• JSON validation: cloud/registry.json parses.
• End-to-end: cookiecutter gh:dotflow-io/templates with each cloud option and inspect generated files (pending release).

Closes #22