Skills: replace Python release helper with Node .mjs

[huntharo]

Summary

Replace the repo-local Python release helper with a Node .mjs script, with zero runtime dependencies if practical.

Why

The repo-local automation preference for skills is already clear:

• prefer Node scripts for repo-local automation
• add repo devDependencies only when necessary
• avoid Python unless it is clearly worth the extra runtime dependency

The current project-manager helper already follows that preference:

• .agents/skills/project-manager/scripts/sync-work-items.mjs

But the release skill still ships a Python helper:

• .agents/skills/release/scripts/release_plan.py

That inconsistency adds an extra runtime/tooling dependency for a repo-local skill when a Node script should be sufficient.

Current State

• project-manager automation is already implemented as Node .mjs
• release automation still depends on release_plan.py
• the release skill relies on that helper to gather release facts and draft raw note inputs

Desired Behavior

• Reimplement the release planner as a Node .mjs script.
• Prefer zero dependencies if the GitHub CLI and built-in Node APIs are enough.
• Keep the behavior and outputs compatible with the current release workflow:
  • writes .local/release/release-plan.json
  • writes .local/release/release-plan.md
  • proposes the next tag
  • groups PR-backed changes and direct commits
  • filters release-housekeeping noise

Acceptance Criteria

• The Python helper is removed or no longer required by the release skill.
• The replacement is a Node .mjs script checked into the repo.
• The release skill docs point at the new script.
• The replacement preserves the current planner outputs closely enough that the release workflow does not need to be redesigned.

Notes

• If one or two small Node dependencies make the implementation materially better, that is acceptable, but the default target should be zero deps.
• This issue is about repo-local skill automation consistency, not a redesign of the release workflow itself.