feat(context branching): add context branching for threads

[William-zgx]

Summary

• implement minimal thread context branching across the backend thread flow and the frontend workspace navigation
• add backend support for branch creation metadata and branch-aware thread operations
• add frontend branch metadata, APIs, route helpers, and sidebar tree rendering for branch navigation

Scope

• this PR is implementation-only
• design discussion is tracked in Proposal: context branching for threads #2098
• the standalone design doc has been removed from this PR

Testing

• backend/.venv/bin/pytest backend/tests/test_threads_router.py -q
• backend/.venv/bin/pytest backend/tests/test_provisioner_kubeconfig.py backend/tests/test_docker_sandbox_mode_detection.py -q
• pnpm --dir frontend typecheck
• pnpm --dir frontend exec node --test src/core/threads/utils.test.ts

[CLAassistant]

All committers have signed the CLA.

[WillemJiang]

@William-zgx, thanks for your contribution. As it's a new feature, please create an issue with all your designs to disucss with the community first.

[Copilot]

Pull request overview

Adds minimal “context branching” support for threads by introducing backend lineage metadata + a branch creation endpoint, then wiring frontend APIs, routing helpers, and sidebar rendering to create and navigate branch threads.

Changes:

• Backend: add POST /api/threads/{thread_id}/branches to fork a new thread from a checkpoint, copy selected thread-local files, and persist branch lineage metadata.
• Frontend: add thread branch APIs/hooks and UI actions (create branch / back-to-parent), plus branch-aware routing.
• Frontend: render recent chats as a parent/child tree using branch metadata.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
backend/app/gateway/routers/threads.py	Adds branch create request/response models, file-copy helpers, LangGraph thread registration helpers, and the /branches endpoint.
backend/tests/test_threads_router.py	Adds unit tests covering branching behavior, checkpoint selection, file copying, and rollback on failure.
frontend/src/core/threads/types.ts	Introduces branch metadata types and request/response types for branching + thread records.
frontend/src/core/threads/utils.ts	Adds agent-name normalization and branch metadata helpers; improves thread path resolution.
frontend/src/core/threads/api.ts	Adds fetchThreadRecord and createThreadBranch API calls.
frontend/src/core/threads/hooks.ts	Adds useThreadRecord and useCreateThreadBranch; updates rename to invalidate thread detail queries.
frontend/src/core/threads/index.ts	Re-exports new thread API and utils.
frontend/src/components/workspace/thread-branch-actions.tsx	Adds UI for creating branches and returning to parent thread.
frontend/src/components/workspace/recent-chat-list.tsx	Builds and renders a branch tree in the sidebar recent chats list.
frontend/src/app/workspace/chats/[thread_id]/page.tsx	Adds branch actions to the default chat header.
frontend/src/app/workspace/agents/[agent_name]/chats/[thread_id]/page.tsx	Adds branch actions + redirects /agents/default/... back to default chat routes.
frontend/src/core/i18n/locales/en-US.ts	Adds branching UI strings (English).
frontend/src/core/i18n/locales/zh-CN.ts	Adds branching UI strings (Chinese).
frontend/src/core/i18n/locales/types.ts	Adds branching translation schema.
frontend/next.config.js	Adds gateway rewrites for additional /api/* endpoints including /api/threads/:path*.

[WillemJiang]

@William-zgx, thanks for your contribution. Please take a look at the review comments.

[William-zgx]

Addressed in d95980f. I added defensive branch-depth parsing, rollback on copy/store failures, skipped symlinks during branch file copy, cleared artifacts when outputs are not copied, aligned the timestamp description, and added the missing /api/threads rewrite.

[William-zgx]

@William-zgx, thanks for your contribution. Please take a look at the review comments.

@WillemJiang Thanks, I’ve addressed the review comments, updated the PR, and resolved the related threads.

[William-zgx]

@WillemJiang could you please take a look when you have a moment? I've addressed the review comments and updated the PR. Thanks!