feat: add user diff tracking to show external file changes between generations

[R44VC0RP]

Summary

This PR implements a "user diff" feature that tracks and displays external file changes made by users between AI generations. When a user modifies files outside of OpenCode and submits a new prompt, the system now shows what changed.

• Captures a snapshot at the end of each successful AI generation
• Computes diff against the snapshot when a new prompt is submitted
• Shows changes in the TUI with file status indicators (A/M/D)
• Injects diff context into the conversation for the AI to understand
• Includes protections against context overload (max 50 files, 10k chars)
• Ignores common non-essential files (node_modules, dist, lock files, etc.)

Changes

• Added lastGenerationSnapshot field to Session schema
• Created new UserDiffPart message part type
• New user-diff.ts module for diff computation and formatting
• Modified SessionProcessor to capture snapshots at end of generation
• Modified SessionPrompt to compute and inject user diffs
• Added TUI rendering for user diff display
• Regenerated SDK types

Design Doc

See specs/user-diff.md for the full design specification.

[R44VC0RP]

this is a POC do not merge