[FEATURE] Add release tag link to CHANGELOG heading during finalize-release

[c-vigo]

Description

The finalize_release_date function in prepare_changelog.py currently replaces ## [X.Y.Z] - TBD with ## [X.Y.Z] - YYYY-MM-DD, but does not add a link to the GitHub Release tag. The link has been added manually in separate post-release commits for every past release (e.g. 7acaab4 for 0.3.1, 0217d7d for 0.3.0).

Problem Statement

Every release requires a manual follow-up commit to transform ## [X.Y.Z] - DATE into ## [X.Y.Z](https://github.com/vig-os/devcontainer/releases/tag/X.Y.Z) - DATE. This is easy to forget and adds unnecessary toil.

Proposed Solution

Update finalize_release_date() in packages/vig-utils/src/vig_utils/prepare_changelog.py to produce the replacement string with the release tag URL:

replacement = f"## [{version}](https://github.com/{repo}/releases/tag/{version}) - {release_date}"

The repo URL should be derived from a parameter or environment variable (e.g. GITHUB_REPOSITORY) rather than hardcoded, so the same tool works for downstream workspaces.

Alternatives Considered

• Add the link in a separate post-finalize step in release.yml using sed. This keeps prepare_changelog.py simpler but scatters changelog logic across shell and Python.
• Add the link during promote-release. The tag exists by then, but it delays the link until after the release is already published as draft.

Impact

• No breaking change. The only difference is the heading format in CHANGELOG.md gaining a link.
• Benefits every release going forward by eliminating a manual step.
• The downstream workspace prepare_changelog would also benefit if the same change is applied to the workspace template.

Changelog Category

Changed

Acceptance Criteria

• finalize_release_date() produces headings with the release tag URL
• Repo URL is configurable (not hardcoded)
• Existing tests updated; new tests cover the link format
• TDD compliance (see .cursor/rules/tdd.mdc)