chore: add prepare-release and publish-release agent skills (#6147)

* chore: add prepare-release and publish-release agent skills

Split the release process from docs/release.rst into two agent skills:
a changelog/version-bump PR, and the manual publication steps after
that PR merges. Un-ignore .agents (the MSVC 'Release' pattern matched
the release skill directory case-insensitively) and drop the outdated
manual _version.py step from docs/release.rst.

Assisted-by: ClaudeCode:claude-fable-5

* [no ci] chore: document changelog style normalization in release skill

Teach the prepare-release skill to proofread generated changelog entries for tense, concision, categorization, and PR-link consistency before opening release-prep PRs.

* [no ci] chore: fix release skill portability

* [no ci] fix: harden release workflows

* [no ci] docs: show explicit manual artifact upload

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Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>