docs(seo): clarify the SEO panel is only honored if the page wires getSeoMeta

[DavidPivert]

What does this PR do?

Adds a prominent gotcha + a "composing panel overrides with custom defaults" snippet to the SEO section of skills/building-emdash-site/references/site-features.md.

The admin SEO panel (SEO title, meta description, OG image, canonical, noindex) lives on entry.data.seo and has no effect unless the page calls getSeoMeta/getContentSeo. The current docs only demonstrate this for a posts page and never warn that hand-written <title>{entry.data.title}</title> silently bypasses the whole panel — including the noindex toggle. This is easy to hit on real sites (no error, no warning).

Changes:

• ## SEO Meta: add an [!IMPORTANT] callout that the panel is only honored if the page reads it, and that every content detail page must wire it.
• New subsection "Composing panel overrides with custom defaults": since getSeoMeta only falls back to data.title/data.excerpt, show how to use getContentSeo to let the panel take precedence over a page's computed title/description.

Docs-only. The same skill is vendored under templates/*/.agents/skills/...; left to whatever sync process keeps them in step with skills/ — happy to update those too if maintained by hand.

Refs #1518

Type of change

• Bug fix
• Feature (requires maintainer-approved Discussion)
• Refactor (no behavior change)
• Translation
• Documentation
• Performance improvement
• Tests
• Chore (dependencies, CI, tooling)

Checklist

• I have read CONTRIBUTING.md
• pnpm typecheck passes — N/A (docs-only, no code changed); CI Typecheck is green
• pnpm lint passes — N/A (docs-only)
• pnpm test passes (or targeted tests for my change) — N/A (docs-only)
• pnpm format has been run — N/A (markdown only)
• I have added/updated tests for my changes (if applicable) — N/A
• User-visible strings in the admin UI are wrapped for translation (if applicable) — N/A
• I have added a changeset (if this PR changes a published package) — N/A (docs only, no published package changed)
• New features link to an approved Discussion — N/A (not a feature)

AI-generated code disclosure

• This PR includes AI-generated code — model/tool: Claude Opus 4.8 (Claude Code)

Screenshots / test output

N/A — documentation change. See #1518 for the motivating real-world case.

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commit: 9e7d243

[DavidPivert]

I have read the CLA Document and I hereby sign the CLA

[DavidPivert]

recheck

[DavidPivert]

Heads-up for a maintainer on the red checks — none are caused by this PR:

• This is a docs-only change (one Markdown file, skills/building-emdash-site/references/site-features.md), so it can't affect runtime behaviour or E2E.
• E2E Tests (required) is red only because it aggregates the shards, and shard 4/8 had 1 failed / 33 passed: invite-flow › "invited user appears in the users list" — a toBeVisible timeout unrelated to this change (looks flaky). A re-run should clear it.
• CLA Assistant / label fails on the "Label CLA status" step — a pull_request_target label write that lacks permissions from a fork. The CLA itself is signed (the CLAssistant check is green).

As a fork contributor I can't re-run Actions — happy for a maintainer to re-run the E2E shard whenever convenient. Thanks!