NEXUS-4022: Start region chats in the New Chat editor - #2784

## Summary

Exposes the bound user's personal Solo Workspaces to plugin regions
(`2cbf04b6d`).

`ctx.soloNavigation.workspaces` takes the current region binding and
offers:
- `snapshot`: the workspace IDs and names, the catalog status, whether
creation is available, and any read error;
- `subscribe` and `refresh`;
- `create({ id, name })`.

It uses the existing Solo preference writer, so that writer's
authorization, capacity and revision checks apply. An ended binding
cannot read or write another community. Ordinary chat filing stays
host-owned. `docs/solo-regions.md` documents the contract.

### Compared with the previous version of this PR

The PR no longer contains "add primary controls and the workspace
header". It is based on #2784's branch instead of #2785's.

buzz-review-completed

## Testing

Local runs on macOS, on the stack rebased onto main `9596cd3db`:
- At this PR's head `2cbf04b6d`: `pnpm check` (Biome, TypeScript and the
design checks) and the icon check passed. Vitest for the files the flake
check selects for this PR: 436 of 438 passed in a run that shared the
machine with a Rust build; the 2 tests that timed out
(`App.profile.test.tsx`, `entity-navigation.test.tsx`) passed on rerun,
52/52 in their files.
- At the stack tip `adbdf424e`: full Vitest, 20556 passed and 23 skipped
(includes `browser-host/channel-kit.test.mjs`).
- Browser journeys at the stack tip, Chromium and WebKit: the full
`tests/browser/solo-rename.spec.mjs`, repeated 5 times, passed 120/120.
The 34 other specs that the flake check relates to the stack, run once
in each engine: 343 passed, 3 skipped by their own conditions. The full
browser suite was not run locally.
- The Nexus plugin's connected browser harness ran on the actual Buzz
web shell and plugin UI, with a simulated relay and generated
identities, before the rebase. Full run at host pin `049a9b57e` (now
`1fbd49c49`), which includes this PR: 53 passed.

**Native testing:** Gil ran the native desktop app on this stack at
`adbdf424e`, with the matching Nexus plugin build. Gil confirmed that
these native-only paths passed:
- `!` folder listing through the Tauri `repositories_folder_list`
command (#2789);
- `/` SKILL.md listing through `local_skills_list` (#2790);
- the Claude `--add-dir` folder grant (#2789);
- relay signing and Outbox retry after a native restart.

Agent testing did not reach these paths: the agent ran no native app, no
real relay and no real agent.

## Known gaps

- None specific to this PR.

Hosted or local failures that this stack does not cause:
- `tests/browser/typing.spec.mjs:290` ("a growing channel/thread
composer lifts history with it at the bottom") intermittently fails with
React error #185 on main too. Locally it failed 12 of 12 runs at
`9596cd3db` and 9 of 12 at a later main commit.
- On macOS, `cargo test -p Buzz` crashes the library test binary
(SIGSEGV), and some `host_command` deadline tests fail. Both happen the
same way at base `9596cd3db`, before this stack. Hosted Linux CI is the
reference for these.

## Test changes

Titles are compared only in the test files each range changes. Tests
that moved or were renamed are matched by title.

<details><summary>Removed by this PR's diff (0)</summary>

- none

</details>

<details><summary>Added by this PR's diff (1)</summary>

- `src/features/channel-navigation/solo-workspaces.test.ts`: projects
the existing Solo catalog and writer, preserving uncertainty and fencing
ended bindings

</details>

<details><summary>In the previous version of this PR, not carried over
(2)</summary>

- `src/features/solo/SoloDraft.test.tsx`: permits one start across
already-open drafts and preserves unresolved payload (%s)
- `src/features/solo/SoloDraft.test.tsx`: shows pending accepted-message
recovery and cancels binding when its draft region ends

</details>

Signed-off-by: Gil Birman <[email protected]>
Co-authored-by: Codex <[email protected]>

* origin/main:
  feat(messages): let attachment renderers open files from the host file card (block#2239)
  feat(charts): bundle a Charts code-block renderer (block#3226)
  Expose native reply grouping through Conversation.ui (block#3245)
  NEXUS-4163: Keep plugin-owned chat deletion in Solo (block#2787)
  NEXUS-4156: Expose Solo Workspaces to plugin regions (block#2786)
  NEXUS-4022: Start region chats in the New Chat editor (block#2784)
  feat(link-previews): unfurl web links below messages by default (block#3225)
  fix(messages): keep names in literal Markdown as written when editing (block#2834)
  feat(plugins): fetch a packaged managed plugin policy from a URL (block#2504)

Signed-off-by: Joah Gerstenberg <[email protected]>

# Conflicts:
#	crates/plugin-manager/tests/management.rs
#	docs/plugin-architecture.md

…y-untagged-reactions

* origin/main: (324 commits)
  fix(github): show private-repo body images from signed links (#3252)
  NEXUS-4029: Preserve semantic chips in the core composer (#2303)
  Make plugin signature enforcement opt-in (#2212)
  NEXUS-4156: Add workspace Leads and choose a chat's workspace (#2788)
  feat(plugins): report conclusively retired update providers (#3263)
  feat(messages): let attachment renderers open files from the host file card (#2239)
  feat(charts): bundle a Charts code-block renderer (#3226)
  Expose native reply grouping through Conversation.ui (#3245)
  NEXUS-4163: Keep plugin-owned chat deletion in Solo (#2787)
  NEXUS-4156: Expose Solo Workspaces to plugin regions (#2786)
  NEXUS-4022: Start region chats in the New Chat editor (#2784)
  feat(link-previews): unfurl web links below messages by default (#3225)
  fix(messages): keep names in literal Markdown as written when editing (#2834)
  feat(plugins): fetch a packaged managed plugin policy from a URL (#2504)
  fix(goose): enable deferred tool loading for goose agents (#3227)
  fix(unread): preserve manual intent in captured explicit reads (#3228)
  NEXUS-4337: Expose retained plugin update reviews (#3075)
  feat(conversation): add a link preview slot below messages (#3219)
  feat(plugins): let link previews read a public page's preview fields (#3217)
  Let sidebar row plugins decorate ordinary Solo chats (#3222)
  ...

* origin/main: (121 commits)
  Expose Solo region state before a component mounts (#3296)
  fix(builderlab): say a 401 was refused, not expired, and show each sign-in failure once (#3241)
  feat(agents): file new Solo chats in explicit personal workspaces (#2857)
  perf(css): limit broad style invalidation (#3267)
  Delay and fade in the Bestie badge celebration (#3292)
  fix(github): show private-repo body images from signed links (#3252)
  NEXUS-4029: Preserve semantic chips in the core composer (#2303)
  Make plugin signature enforcement opt-in (#2212)
  NEXUS-4156: Add workspace Leads and choose a chat's workspace (#2788)
  feat(plugins): report conclusively retired update providers (#3263)
  feat(messages): let attachment renderers open files from the host file card (#2239)
  feat(charts): bundle a Charts code-block renderer (#3226)
  Expose native reply grouping through Conversation.ui (#3245)
  NEXUS-4163: Keep plugin-owned chat deletion in Solo (#2787)
  NEXUS-4156: Expose Solo Workspaces to plugin regions (#2786)
  NEXUS-4022: Start region chats in the New Chat editor (#2784)
  feat(link-previews): unfurl web links below messages by default (#3225)
  fix(messages): keep names in literal Markdown as written when editing (#2834)
  feat(plugins): fetch a packaged managed plugin policy from a URL (#2504)
  fix(goose): enable deferred tool loading for goose agents (#3227)
  ...

Signed-off-by: Tyler Longwell <[email protected]>
Co-authored-by: Eva <011987e296fd5006292d2f930b574be47c7801048d1983c46c425d3c95f0cffd@buzz.block.builderlab.xyz>