Let the seed and the CLI set an entry's publication date (publishedAt)

[itaides]

The problem

A migration can't carry an entry's original publication date through the seed or the CLI, so every imported entry says it was published on the day of the import.

We moved a WordPress site with ~680 posts (2016–2026) to EmDash. After seeding, all 681 entries had published_at = the import day, so the journal was ordered by import time, the RSS feed's pubDates and the sitemap's lastmod were all the same date, and datePublished in the JSON-LD was wrong for every post.

• SeedContentEntry (packages/core/src/seed/types.ts) has status, data, taxonomies, bylines, locale… but no date, and status: "published" stamps "now".
• emdash content create / update take --data, --slug, --locale, --translation-of, --draft — no date. content schedule --at only accepts a future time.
• The REST API does accept publishedAt on a write, and the WordPress importer (cli/commands/import/wordpress.ts) already maps pubDate to publishedAt. So the column and the write path exist; only the seed format and the CLI don't expose them.

We worked around it with a script that writes published_at straight into the database after seeding (and again over wrangler d1 execute for D1, which then also needs the KV content cache cleared by hand). That's the part every migration would rather not write.

Proposal

Small and additive:

1. SeedContentEntry.publishedAt?: string (ISO 8601). Honoured only with status: "published"; ignored for drafts. Optionally createdAt? too, for the same reason.
2. emdash content create|update --published-at <iso> that sends the existing publishedAt field.
3. Past dates allowed (that's the point); a future date keeps meaning "scheduled", as today.

Stored as UTC like everything else since the 1.0 datetime normalization.

Happy to open the PR if this direction is OK.

Drafted with Claude (Opus 5.5) from notes on our migration; checked against main at 584f166.