feat: collapsible JSON tree view for raw messages in context panel

Description

The raw messages section in the context panel (side panel → context tab → "Raw messages") currently renders each message as a flat JSON text block using a code viewer. This makes it difficult to read deeply nested JSON structures — users have to scroll through hundreds of lines to find the relevant field.

Solution

Replace the plain text JSON display with a collapsible tree view that allows users to expand/collapse individual nodes (objects and arrays). This makes the raw data much more scannable and useful for debugging.

Key behavior:

• Top-level nodes are expanded by default; nested children are collapsed
• Each expandable node shows a chevron-right toggle button that rotates 90° when expanded
• Collapsed objects/arrays show a { ... } / [...] summary
• Syntax coloring: keys (magenta), strings (teal), numbers (cyan), booleans (orange), null (gray)
• Gracefully falls back to raw text if JSON parsing fails

Implementation

• packages/session-ui/src/components/json-viewer.tsx — recursive tree component
• packages/session-ui/src/components/json-viewer.css — syntax highlighting via existing --syntax-* CSS variables
• packages/app/src/components/session/session-context-tab.tsx — RawMessageContent updated to use JsonViewer instead of <File mode="text">

No new external dependencies. Uses existing @opencode-ai/ui/icon and theme variables.

Screenshots

N/A — pure UI change, no visual mockup needed.

Additional context

This is a self-contained enhancement. A follow-up could add the same treatment to diff/file views, but that requires deeper integration with @pierre/diffs and is scoped separately.