v0.62.1: in-admin editor help

This release closes the editor-help initiative, three passes that give a non-technical author help where they work. It rolls up 0.61.0, 0.62.0, and 0.62.1; the two intermediate versions were never published on their own. No consumer action is required.

0.61.0 laid the foundation. A frontmatter field can now carry a description, one author-facing sentence shown under the field in the editor's Details panel and tied to the input with aria-describedby. The date field ships a built-in publish-clarity hint, so a new site reassures the author that publishing is a separate step without any per-field copy to write. An adapter can set a supportContact that the runtime carries through to the admin.

0.62.0 added the Help home, a standing screen at /admin/help reached from a pinned Help link at the foot of the office sidebar and from the Ctrl+K command palette. A getting-started checklist derives its progress from the real corpus, writing a post, publishing one, and creating a page, and the section drops away once all three are done. A formatting reference promotes the editor's Ctrl+/ cheat sheet to a standing two-column table. A support hand-off points a stuck author at the site's supportContact and renders only when the adapter sets one.

0.62.1 gave the editor an advisory channel and its first notice. When another entry already resolves to the same public address, the editor shows a non-blocking warning that names that entry and links to it, which makes the last-write-wins outcome visible instead of silent. The check runs at edit-load across main and every open cairn/<concept>/<id> branch, and a publish re-checks the address and emits a publish.address_collision log event when it overrides one. The existing needs-alt notice now renders through the same advisory region.

The window adds HelpData, AdvisoryNotice, and AdvisoryAction on /sveltekit, the HelpHome component on /components, and the additive /admin/help route. Per-version detail is in CHANGELOG.md.