Prepare 0.15.0 release tooling, site metadata, and release-prep docs

[chalin]

• Aligns the repo with Hugo 0.157.0 using a small set-hugo-version helper and maintainer notes so pins stay consistent and easier to update.
• Sets package / prerelease versioning to 0.15.0-dev so the tree clearly tracks the upcoming minor before the tag.
• Fixes get-build-id so semver from package.json is not overridden by an older tag-derived version when the package is already ahead (avoids wrong build/version IDs in shipped metadata).
• Writes build/version-related params on docsy.dev so local and CI builds reflect the intended identifiers.
• Refreshes 0.15 release-prep task docs (plan, issue audit/mapping, inventory, wrapup) so milestone work and channel status stay accurate for reviewers.
• Tweaks published-facing copy (0.15 blog post, changelog snippet, agent-support page, spelling/copyedits) so the release narrative matches the branch.
• Updates refcache.json where link checks require it.