⚙️ FEATURE-#269: Managed DotFlow with name, idempotent start, and CLI cleanup

[FernandoCelmer]

Summary

Follow-up to merged PR #271. Tightens the managed/cloud integration pieces around dotflow login.

• DotFlow gains name (defaults to socket.gethostname()) and an idempotent .start() that swallows duplicate calls when the task queue is unchanged.
• SerializerWorkflow / SerializerTask now expose id/name as @computed_field mirrors of the internal workflow_id/task_id/workflow_name — API compatibility without duplicating storage.
• ServerDefault reads from ~/.dotflow/config.json (env wins) and uses the new /cli/workflows/* endpoints.
• dotflow login dropped --token and --base-url flags (credentials on argv is a security footgun).
• Stray breakpoint() removed from dotflow start --workflow path.
• hostname() helper moved into dotflow.utils.
• New how-to docs for dotflow login / dotflow logout; ServerDefault page updated.
• Test coverage: test_serializer_workflow.py, idempotent start scenarios, flag-less login assertions.

Test plan

• poetry run pytest tests/
• poetry run ruff check + ruff format --check
• Manual: dotflow login stores config, dotflow start --workflow x:main runs tasks once