Document the BYOK AI Agent + new PluginAPI verbs/tiers

[kipavy]

Document the BYOK AI Agent + new PluginAPI verbs / tiers

Tracks documentation for the AI agent feature.

Related epic: VoltiusApp/voltius#55

Work

• User-facing AI Agent page (setup: provider + BYOK key, model choice; the
  chat panel, command-palette entry, terminal shortcut; approval modes incl. plan /
  auto-accept).
• Plugin API reference updates: new verbs (sftp, settings.get, session
  open/close, workspace/layout; gated: terminal-output read, plugins.install,
  security settings.set).
• Document the public vs gated (first-party-only) capability-tier concept and
  the Node-less-webview rationale.
• Note that gated verbs are not available to marketplace plugins (link
  marketplace policy issue).