ci: switch releases to SemVer VERSION workflow

[aoirint]

Warning

This pull request was created with assistance from LLMs.

Summary

• Replaces the current release workflow with the aoirint/comfyui_docker style VERSION + Git tag comparison flow.
• Adds VERSION with 0.0.0, removes PEP 440/date-version release generation, and leaves pyproject.toml at the inert project version 0.0.0.
• Treats VERSION=0.0.0 as an edge-only build so it does not create a GitHub Release.
• Publishes stable Docker images as v<VERSION> and latest, prerelease/edge builds as edge, and switches build cache tags to buildcache / edge-buildcache.
• Removes the old local release metadata composite actions and documents the new release procedure in README.md.

Related Issues

• Refs build: align uv project configuration #79
• Refs https://github.com/aoirint/comfyui_docker/tree/745a6a033b2269fa6f6b2fc400fada67807f215e

Notes for reviewers

This PR prepares the release workflow without publishing a release version. With VERSION=0.0.0, pushes to main are treated as edge builds until maintainers intentionally set a releasable SemVer value.

AI disclosure

Codex helped compare the existing workflow with aoirint/comfyui_docker, adapt the release flow to this repository, resolve the post-#79 merge conflict, run workflow/package checks, and draft this pull request body.

Testing

Automated checks

• actionlint -pyflakes=
• pinact run --check --min-age 7
• uv lock --check
• git diff --check

Manual checks

• Confirmed VERSION is 0.0.0 and matches the workflow SemVer pattern.
• Confirmed the workflow explicitly maps VERSION=0.0.0 to release_mode=edge, so it will not create a GitHub Release.

AI-assisted inspections

Request: Check the release workflow change for GitHub Actions and security-sensitive publishing concerns.
AI-assisted result: The workflow keeps actions pinned to full commit SHAs, uses job-scoped permissions, limits release write permission to the release job, and keeps Docker publishing credentials scoped to the build job.