feat(seo): getSeoMeta accepts defaultTitle/defaultDescription (#1518)

[swissky]

What does this PR do?

Implements suggestion 2 from #1518 (milestone 1.0): getSeoMeta now accepts defaultTitle and defaultDescription options.

Design discussion: #1956

Today the helper's only fallbacks are data.title / data.excerpt. A page with a computed title (the issue's example: "Maeta — {title} (cover of {artist})") can't use getSeoMeta at all — the author has to drop down to getContentSeo and merge panel-vs-default by hand, or (worse, and this is the footgun) hand-roll <title> and silently bypass the whole SEO panel including the noindex toggle.

The new options slot in between the panel value and the data fallback:

• title: SEO panel title > defaultTitle > data.title
• description: SEO panel description > defaultDescription > data.excerpt

Editor-set panel values always win, so the panel stays authoritative. Purely additive; existing callers are unaffected. The siteTitle suffix applies to a default title like any other page title.

Complements the docs fixes for suggestion 1 (#1519 by the reporter, #1900) by removing the main remaining reason to hand-roll <title> in the first place. Suggestion 3 (dev-mode warning) is intentionally out of scope — it needs render-path tracking and is better evaluated once docs + this helper have landed.

Refs #1518

Type of change

• Bug fix
• Feature
• Refactor
• Translation
• Documentation
• Performance
• Tests
• Chore

How was this change tested?

• New unit tests in tests/unit/seo/get-seo-meta.test.ts: defaults beat data.title/data.excerpt, panel values beat defaults, behavior without defaults is unchanged, siteTitle suffix applies to the default title
• Existing tests/integration/seo/seo.test.ts still passes (67 tests total across both files)
• pnpm typecheck, pnpm lint, pnpm format all clean

Checklist

• I have read CONTRIBUTING.md
• I have added tests that prove my fix/feature works
• New and existing tests pass locally
• I have added a changeset (pnpm changeset) if this change affects published packages
• For features: I have opened a Discussion and linked it here

AI-generated code disclosure

• Portions of this PR were generated with an AI tool

Which tool? Cursor (Fable 5)

[changeset-bot]

🦋 Changeset detected

Latest commit: 54caa74

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 16 packages

Name	Type
emdash	Minor
@emdash-cms/cloudflare	Minor
@emdash-cms/sandbox-workerd	Patch
@emdash-cms/fixture-perf-site	Patch
@emdash-cms/perf-demo-site	Patch
@emdash-cms/cache-demo-site	Patch
@emdash-cms/do-demo-site	Patch
@emdash-cms/do-solo-demo-site	Patch
@emdash-cms/admin	Minor
@emdash-cms/auth	Minor
@emdash-cms/blocks	Minor
@emdash-cms/gutenberg-to-portable-text	Minor
@emdash-cms/x402	Minor
create-emdash	Minor
@emdash-cms/auth-atproto	Patch
@emdash-cms/plugin-embeds	Patch

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commit: 54caa74

[emdashbot]

This is the right change, implemented in the right place. getSeoMeta is the helper callers already reach for, and adding defaultTitle/defaultDescription as an extra rung in the existing fallback ladder solves the computed-title use case without making anyone drop down to getContentSeo or hand-roll <title>. It is purely additive: the return type is unchanged, the new options are optional, and the precedence (panel value → caller default → content field) matches the PR description exactly.

I checked the helper implementation, the new unit tests, the changeset, and package exports. The precedence chain is correct, siteTitle suffix behavior is preserved, and the fallback to data.title/data.excerpt is unchanged when no defaults are supplied. The changeset is user-facing, present tense, and identifies the migration story. No AGENTS.md conventions are violated: there is no new user-facing UI text, no SQL, no auth/locale/database changes, and a changeset is included.

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