Embed a Monaco-backed file viewer and diff review surface

[KSemenenko]

Problem

The workbench needs a serious file surface for inspection, editing, and diff review instead of a static chat-only experience.

Scope

• Add file viewing, syntax-aware editing, read-only mode, and diff review mode
• Define how agent-proposed edits remain reviewable before acceptance
• Cover file open and compare flows from the workspace tree and session surface

Out of scope

• Full IDE parity beyond the workbench goals
• Git backend implementation details outside viewer and diff requirements

Implementation notes

• Use Monaco as the preferred editor surface
• Keep diff review explicit and approval-friendly
• Align file-surface behavior with the Git and session review issues

Definition of Done

• The issue defines read-only, user-edit, and agent-proposed diff modes
• The review surface requirements are specific enough for UI and runtime work later

Verification

• Review the issue body against the feature spec review flow

Dependencies

• Parent epic: [Epic] Replace the static prototype with a real desktop workbench #13