Public changelog from completed items #23

Problem

The roadmap shows what is planned and in progress, but Completed items just sit in a column with no release context. Users cannot see what shipped, when, or in which version. A public changelog is the natural second half of a roadmap, closes the loop for voters ("you asked, we shipped"), and doubles as SEO content.

Proposal

Generate a public changelog from items marked Completed, grouped by release.

Data

• New optional item fields (shown in the item editor sidebar): Released on (date) and Version / release name (text, e.g. v2.4).
• When an item is moved to a status flagged as "done" (new status option "Counts as shipped", default on for Completed), auto-fill Released on with today's date if empty.
• Optional free-form "Release notes" excerpt (falls back to item content).

Front end

• [niroroadmap_changelog] shortcode and Changelog block. Attributes: product, limit, group="month|version", tag.
• Reverse-chronological feed: release header (version/date), list of shipped items with tag chips and vote count ("Requested by 128 people").
• Each entry opens the same popup, or links to the item.
• Optional RSS/Atom feed at /niroroadmap/changelog/feed/ and <link rel="alternate"> in the page head.
• Schema.org Article/ItemList markup (optional).
• Link between board and changelog (a "See what shipped" link in the Completed column; setting).

Admin

• Changelog page picker in Settings (auto-create like the Roadmap page, once).
• Bulk action "Mark as released in version..." on the items list.

Acceptance criteria

• Items in a "shipped" status appear in the changelog, ordered by release date.
• Grouping by month and by version both work; items with no version fall back to month.
• Per-product filtering works.
• Shortcode and block render identical output and are cache friendly.
• Feed validates in a feed validator and only contains published items.
• Drafts, pending, and password-protected items never appear.
• Output is escaped; works with default block themes and classic themes.
• Existing Completed items without a release date show under "Earlier".

Technical notes

• Store as post meta released_at, release_version; expose via REST for the block editor.
• Reuse the CSS custom property system for styling.
• Hook for the notify issue: send the "it shipped" email at the same moment as the release date is set.

Priority rationale

Medium. Strong value and cheap to build on existing data; not blocking other work.