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

We were incorrectly wrapping new terminal auth methods in double ssh
calls.

Only affected ACP beta users, but important for testing and stabilizing
the feature.

We moved the ssh wrapping to be only added in the acp process creation
where it was needed.

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:

- N/A

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>

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>