feat(chat): add thread revert support across providers

[gianpieropa]

Summary

Introduce a hybrid revert system that restores both workspace files and provider conversation state to any prior user message in a chat session.

This change:

• adds a git-backed checkpoint store capturing workspace state before and after each chat run
• adds backend commands to surface revertable targets and execute thread revert
• wires provider-specific rollback support for Claude, Codex, and OpenCode
• exposes revert on eligible user messages
• updates E2E coverage and developer docs for the revert model

Details

Revert behavior

• Revert is attached to the user message
• Reverting restores the workspace to the checkpoint captured before that prompt
• The reverted prompt, its assistant response, and all later turns are removed from local history
• Revert availability is per run, not global to the session

Implementation

• Added checkpoints.rs, a git-backed snapshot store capturing checkpoint_before / checkpoint_after for chat runs
• Added get_revert_targets and revert_to_message Tauri commands
• Registered the new commands in both src-tauri/src/lib.rs and src-tauri/src/http_server/dispatch.rs
• Surfaced revert in MessageItem and ChatWindow

Provider support

• Claude: uses --resume-session-at / --fork-session rewind anchors
• Codex: uses thread/rollback for the discarded turn count
• OpenCode: uses native session/:id/revert with the parentID anchor

Compatibility

• Older chats still load normally
• Older turns created before checkpoint metadata usually do not show the revert action
• New turns created in older chats can still become revertable

Tests

• Added e2e/tests/revert.spec.ts
• Extended E2E mock fixtures with dynamic handlers for:
  • get_revert_targets
  • revert_to_message

Docs updated

• docs/developer/architecture-guide.md
• docs/developer/data-persistence.md
• docs/developer/testing.md

Validation

• bun run lint
• bun run typecheck
• cargo check --manifest-path src-tauri/Cargo.toml

Manual testing

• Created fresh Claude chat sessions with multiple turns and verified reverting from a user message removes that prompt and newer turns, while restoring files to the pre-prompt state
• Created fresh Codex chat sessions with multiple turns and verified revert works correctly without runtime panics
• Created fresh OpenCode chat sessions with multiple turns and verified native provider rollback works together with Jean file restore
• Verified revert is only shown on eligible user messages
• Verified older pre-feature turns do not show revert
• Verified newer turns created in existing chats can become revertable
• Verified the confirmation dialog appears before revert and uses the new checkpoint warning copy

Notes

bun run check:all is still blocked by existing repo-wide Rust clippy failures unrelated to this change.