feat(app): add file tree panel

[edoedac0]

Summary

Closes #10239

• Adds a collapsible file tree panel to the session layout for browsing project files
• Implements directory listing and lazy-loading via the SDK's file API
• Persists file tree state (opened/closed, width) across sessions
• Integrates with file watcher to auto-refresh directories when files are added/removed

Files.system.opencode.mov

Changes

• packages/app/src/context/file.tsx - Added tree state management with listDir, expandDir, collapseDir, and children methods
• packages/app/src/context/layout.tsx - Added fileTree layout state for panel visibility and width
• packages/app/src/components/file-tree.tsx - Updated file tree component
• packages/app/src/pages/layout.tsx - Integrated file tree panel into the session layout
• packages/app/src/pages/session.tsx - Minor session page updates for file tree integration

[github-actions]

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2. Add Fixes #<number> or Closes #<number> to this PR description

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[github-actions]

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