Add a recommended feature set (Yoast-replacement baseline)

### Description

For v2.0, each feature is documented on its own merits and sites choose what they want -- there is no "recommended" list. See `docs/adr/0010-feature-taxonomy.md`.

This issue covers adding a recommended feature set for v2.1, framed as a **Yoast-replacement baseline**: not a curated shortlist, but a migration checklist whose purpose is to stop a site moving off Yoast from silently losing behavior it already had.

The five identified during the v2.0 taxonomy work, with the reason each earns a place:

| Feature | Handle | Why |
|---|---|---|
| Title Tags | `titles` | Core SEO output |
| Meta Descriptions | `descriptions` | Core SEO output |
| Canonical URLs | `canonical_urls` | Duplicate-content protection |
| Robots Meta | `robots_meta` | Basic hygiene -- noindex on search results and 404s |
| Open Graph | `open_graph` | Link unfurling; a site silently losing this is the incident that motivated code-only configuration |

Deliberately **not** in the baseline: Arbitrary Meta Tags (`arbitrary_tags`), which is a power-user escape hatch for verification tags and one-offs rather than something a Yoast site would have been relying on.

This list predates the migration of any of these features and should be revisited once they exist, and once any features added after v2.0 (sitemaps, AI meta descriptions, schema) are candidates for it.

Worth considering alongside the docs work: whether the baseline is only documentation, or whether it should also be expressed in code -- a single filter or helper that enables the set, so a site does not have to copy five `add_filter()` calls and keep them in sync with the plugin's recommendations.

### Use Case

Give sites migrating from Yoast a clear "enable these" checklist, so the opt-in default does not translate into silently missing SEO output. Deferred from v2.0 because the features it names do not exist yet.

Start with docs/adr/0010-feature-taxonomy.md and compare its feature handles with the five proposed baseline entries. Define the v2.1 migration checklist and explicitly resolve whether it is documentation-only or also needs a shared code configuration mechanism; the result should cover the five named features, exclude arbitrary_tags, and explain how the list will be revisited.

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