refactor(app): extract context usage extension

[Hona]

Context as a built-in extension

• Move the context-usage button, statistics, system-prompt display, raw messages, and export implementation into @opencode/plugin-context-desktop.
• Declare an initially closed panel and open it from the extension's header action or command. Use the host's closing policy without coupling to another plugin's default panel.
• Read session history and model/provider metadata from the shared public data cache; export every message page through the public client.
• Compose shared controls, Markdown, and file rendering. No imports from App, Desktop, Core, or Server internals.

flowchart LR
  Cache[Public session and model cache] --> Context[Context extension]
  Context --> Button[Session header action]
  Button --> Panel[Host-owned Context panel]
  Context --> Export[Public client: paged session export]

Loading

<Panel id="context" reference="context" initial="closed" title={title}>
  <SessionContextTab session={session} view={session.services.view} />
</Panel>

Before: review-extension	After: context-extension

Synthetic session fixture; no live-session data.

Production session-switch comparison

Baseline review-extension (37218177cf), candidate 4f1dbd8189. Production web renderer, Chromium 147, 400 messages/session, two serial samples per scenario. Median milliseconds; exploratory sample sizes, not a performance-improvement claim.

Scenario	First correct: before	After	Stable: before	After
cold, review closed	293.20	237.35	320.05	255.80
cold, review open	367.75	387.00	403.15	419.30
warm, review closed	64.80	60.50	118.55	121.45
warm, review open	134.35	139.40	163.55	219.20
warm, review resized	174.25	160.25	263.75	234.10