Merge upstream zed-industries/zed main branch (7544515) into Helix fork.

Notable upstream changes:
- Agent V2 feature flag removal (zed-industries#52792)
- Sidebar/MultiWorkspace rework (zed-industries#53096, zed-industries#53032, zed-industries#53267)
- Agent panel git worktree/branch pickers (zed-industries#52979)
- Language model crate refactor (zed-industries#53270)
- Notification panel removal (zed-industries#50204)
- Subagent permission requests (zed-industries#53428)
- Worktree archival on thread archive (zed-industries#53215)
- Agent layout onboarding (zed-industries#52940)
- AuthRequired status for context servers (zed-industries#53396)

Conflict resolutions:
- crates/zed/Cargo.toml: kept external_websocket_sync feature + upstream track-project-leak
- crates/agent_ui/Cargo.toml: kept external_websocket_sync feature + upstream audio feature
- crates/agent_ui/src/agent_panel.rs: preserved all cfg-gated WebSocket blocks,
  migrated should_render_onboarding to should_render_new_user_onboarding,
  added upstream agent_layout_onboarding
- crates/agent/src/tools/context_server_registry.rs: added AuthRequired to match arm,
  preserved pending_tool_loads tracking
- crates/workspace/src/workspace.rs: added new upstream re-exports
- crates/title_bar/src/title_bar.rs: preserved Helix connection status indicator,
  dropped always-hidden onboarding banner (upstream removed it too)
- crates/recent_projects/src/dev_container_suggest.rs: kept suggest_dev_container check +
  upstream cli_auto_open
- .github/workflows, assets/keymaps, assets/settings, editor, multi_workspace: took upstream

Spec-Ref: helix-specs@e637a591b:001723_merge-latest-zed

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- (preview only) sidebar: Fixed issue where tool confirmation indicator
would not show up when subagent asks for permissions

Cherry-picked PRs (in order applied):

1. #53386
2. #53400
3. #53396
4. #53428
5. #53356
6. #53215
7. #53429
8. #53458
9. #53436
10. #53451
11. #53454
12. #53419
13. #53287
14. #53521
15. #53463
16. #52848
17. #53544
18. #53556
19. #53566
20. #53579
21. #53575
22. #53550
23. #53585
24. #53510
25. #53599
26. #53099
27. #53662
28. #53660
29. #53657
30. #53654


Release Notes:

- N/A

---------

Co-authored-by: Danilo Leal <67129314+danilo-leal@users.noreply.github.com>
Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>
Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>
Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>
Co-authored-by: Nathan Sobo <nathan@zed.dev>
Co-authored-by: Anthony Eid <anthony@zed.dev>
Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>
Co-authored-by: Eric Holk <eric@zed.dev>
Co-authored-by: Anthony Eid <hello@anthonyeid.me>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Cameron Mcloughlin <cameron.studdstreet@gmail.com>
Co-authored-by: Cole Miller <cole@zed.dev>
Co-authored-by: Mikayla Maki <mikayla@zed.dev>
Co-authored-by: Anthony Eid <56899983+Anthony-Eid@users.noreply.github.com>
Co-authored-by: Katie Geer <katie@zed.dev>
Co-authored-by: ojpro <contact@ojpro.me>