feat: add improved diff preview for ACP agent file edits

[Henry-916]

Summary

Adds real-time diff preview when ACP agents edit files, using VS Code's native diff editor with a custom URI scheme.

Inspired by Cline's DiffViewProvider architecture. Addresses the issues identified in PR #37 (race conditions, temp file leaks, resource cleanup).

Changes

New files

• src/handlers/DiffContentProvider.ts (20 lines) — TextDocumentContentProvider for acp-diff: custom URI scheme. Old file content is base64-encoded in the URI query parameter, eliminating temp files entirely.
• src/handlers/DiffPreviewHandler.ts (395 lines) — Core diff preview logic:
  • Listens to session/update (tool_call / tool_call_update) notifications
  • Synchronous snapshot on pending/in_progress status — uses fs.readFileSync (no async race condition)
  • Opens vscode.diff on completed status
  • 4-level file path extraction: locations → content[diff] → rawInput key scan → title regex
  • Decoration highlighting for active edit locations
  • Proper dispose() with removeListener + decoration cleanup

Modified files

• src/extension.ts (+12 lines) — Registers AcpDiffContentProvider and DiffPreviewHandler with proper lifecycle management via context.subscriptions.

Design decisions

Decision	Rationale
Custom URI scheme (acp-diff:)	No temp files, no cleanup needed, no file path collisions
Synchronous fs.readFileSync	Eliminates the race condition in PR #37 where async cache could read post-edit content
MD5 hash prefix in URI path	Prevents collisions between same-named files in different directories
isEditKind filter (edit/delete/move only)	Avoids false positives from read/search/think/execute tool calls
tryFinishWithDiffContent fallback	Handles agents that don't send pending status but include diff content in completed

Comparison with PR #37

Issue	PR #37	This PR
Race condition	🔴 Async cacheOldContent can read post-edit content	✅ Synchronous fs.readFileSync
Temp files	🔴 Writes to os.tmpdir(), never cleaned	✅ Zero temp files (URI-encoded)
File name collision	🔴 path.basename only	✅ MD5 hash prefix
Resource leak	🔴 dispose() doesn't removeListener	✅ Full cleanup
Type safety	🔴 (update as any)	✅ ACP SDK types

Known limitations (follow-up)

• Non-UTF-8 encoding: currently assumes UTF-8 (could add iconv-lite detection)
• Large files (>200KB): base64 URI may exceed limits (could fall back to temp file)
• No streaming diff yet (shows only on completed, not during edit)

Testing

1. npm install && npm run compile — compiles cleanly
2. Press F5 in VS Code to launch Extension Development Host
3. Connect to any ACP agent (e.g., hermes acp)
4. Ask the agent to edit a file — diff view should open automatically

Closes #37

🤖 Generated with Claude Code and Reasonix