Fix Cmd+K entity search not finding posts in Commands mode

[KarunyaChavan]

This PR fixes the Cmd+K command palette so typing a post or page title in Commands mode shows matching results you can click or press Enter to open in the editor - matching the behavior of Classic Admin Mode's Cmd+K.

Closes #411

Why?

• In Classic Admin Mode, pressing Cmd+K and typing a post title instantly showed results and let you open the post editor with one click.
• Desktop Mode's Cmd+K showed "No commands matching…" instead.
• Users switching to Desktop Mode expect the same quick-content-search muscle memory to work.
• The REST API search existed but the results were never fetched, displayed, or made clickable.

What changed?

• New REST search (_fetchRemoteCommands): When you type plain text in Commands mode, the palette now calls /wp/v2/search (200ms debounced) and turns matching posts/pages into clickable command items.
• Click handler rewrite: Previously the click handler used findCommand(slug) which only finds registered slash-commands. Entity search results aren't registered commands, so clicks did nothing. Now it looks up by data-index from the live match list instead.
• Result styling: Entity results get a regular font (not the monospace used for slash-commands) via the new .is-entity-result CSS class, so they visually read as content search results.
• Admin URL config: Added adminUrl to the assistant config so entity result links resolve to absolute wp-admin/post.php?post=N&action=edit URLs for opening in legacy windows.

Steps for Testing:

1. Press Cmd+K to open the command palette (make sure you're in Commands mode, not Ask AI)
2. Type the title of an existing post or page - matching results should appear below the command list
3. Click a result - it should open the post/page editor in a legacy window
4. Switch to Ask AI mode and type a natural language question like "How do I create a post?" - pressing Enter should submit the question to the AI, not open a random post

[KarunyaChavan]

We implemented entity search (posts/pages) in the Cmd+K palette — type a title, see results, click to Edit. This closes the gap with Classic Admin Mode.

One detail we didn't replicate: Classic mode shows "Edit" as a right-aligned badge on each result row. We left it out because:

• Each result already shows "Post" or "Page" as a description line below the title
• Entity results already get a different font (regular vs. monospace for slash-commands) via CSS
• Adding a third visual indicator felt like clutter

Would you like us to add "Edit" text anyway to match classic mode exactly? Or is the current approach (description + font difference) sufficient?

[AllTerrainDeveloper]

Would you like us to add "Edit" text anyway to match classic mode exactly? Or is the current approach (description + font difference) sufficient?

We don't necessarily need to replicate everything, if we can improve it :) That's what you did :) Thank you!

And because I tested yours, I found a bug of mine :)

I'll quickly fix the theming issue, which is not applied in the command palette