chore(release): adopt calver release workflow

[ThaddeusJiang]

Summary

• adopt CalVer YYYY.M.PATCH for future save_it releases and document the convention
• remove the v prefix assumption from new release tags and workflow parsing
• add a project-specific save-it-release skill and show the release tag in manual workflow job names

Changes

• update AGENTS.md, CHANGELOG.md, and the changelog skill to describe the new CalVer scheme
• add .agents/skills/save-it-release/ with release guidance and helper scripts for release-state checks and changelog note extraction
• update GitHub release workflows to use tag values directly and show release (<tag>) / docker (<tag>) in the Actions UI

Validation

• ran shell syntax validation for the new release helper scripts
• manually exercised the helper scripts against the current repository state
• did not run app tests because this PR only changes docs, skills, and GitHub workflow metadata/logic