Rewrite TFC module for service workspaces under tfe_project

[filippo-20tab]

Summary

Follow-up to #84. Aligns the generated TFC module with the canonical Minos workspace layout (piano TG-1175 §6, §7).

Changes

• TFC project: introduce tfe_project.main with default_execution_mode = "local". All service workspaces live under it via project_id.
• Workspace naming: ${project_slug}_${service_slug}_${env_name} (e.g. gs1-one_frontend_development). Drop the legacy base/cluster workspaces per stack.
• Drop tfe_workspace_settings: execution mode is now inherited from the project, no per-workspace override needed.
• create_project flag (default true): when Talos has already created the project, the module reads it as a data source instead.
• TFE provider: bump ~> 0.53 → ~> 0.70.
• Runner: new field terraform_cloud_project_create: bool = True; passes TF_VAR_create_project; switches TF_VAR_environments payload from env slugs to env names (matches workspace naming).
• stacks variable removed from terraform-cloud/variables.tf.

Notes

• Existing TFC organizations created by Feature/1175 add minos support #84 will need their pre-existing per-stack workspaces removed by hand — this PR no longer manages them.
• Pairs with the same convention being adopted in talos and django-continuous-delivery.