feat(desktop): add agent browser pane

[Hona]

Summary

• add an advanced-setting-gated native browser pane beside sessions
• expose semantic browser tools only while the exact legacy Session is attached
• bridge the bundled local server to Electron main over the existing utility-process channel
• isolate page content in a sandboxed WebContentsView with permission, navigation, cancellation, and document-generation guards

Scope

This draft targets the current dev/V1 session runtime and bundled local sidecar. WSL and remote servers require the planned ticketed BrowserHost WebSocket and server-network tunnel; they are not claimed by this PR.

Verification

• repository-wide typecheck passed in the push hook
• desktop production build passed
• focused core, tool-schema, registry, layout, and browser-policy tests passed
• session performance unit baseline remained 43/43 passing
• changed-file lint and git diff --check passed