docs: correct the AI Copilot and settings tab references

[mmtr]

Proposed changes

• Rewrite the AI Copilot section of AGENTS.md against the Abilities API. It still documented includes/ai-copilot/tools-registry.php and desktop_mode_register_ai_tool(), both removed in 0.9.4.
• Drop desktop_mode_ai_tool_registered from the listed observability actions. The other nine filters and four actions are live and unchanged.
• Correct the built-in settings tab orders in four places: AGENTS.md, docs/javascript-reference.md, includes/settings-tabs.php, and src/desktop.ts. ai=20 no longer exists and themes=12 was missing.

Why are these changes being made?

AGENTS.md is what agents and contributors read before touching a surface, and docs/ is the public contract for plugin authors. Both pointed at a registration API that was deleted, so anyone following them would register a tool the Copilot never sees. docs/migration-ai-connectors.md already documented the move correctly, the other references never caught up.

The tab list had the same problem in the public registerSettingsTab contract and in two docblocks, so a plugin author picking an order value was working from a list that no longer matches the shell.

Testing instructions

Docs and comments only, no runtime change. To verify the claims:

1. Confirm the removed API is gone: grep -rn "desktop_mode_register_ai_tool\|tools-registry" includes/ src/ returns nothing.
2. Confirm the abilities path in includes/ai-copilot/abilities.php: abilities register on wp_abilities_api_init (line 371), the desktop-mode category on wp_abilities_api_categories_init (line 46), and desktop_mode_ai_search_ability_names() selects on meta.annotations.readonly.
3. Confirm the tab orders against src/settings/panel.ts: appearance=10, themes=12, apps-icons=22, features=25, effects=27, help=40 (admin-only), About pinned with Number.MAX_SAFE_INTEGER.
4. Run npm run build, npm run typecheck, and npm run lint. All pass, and assets/js/ shows no diff.