docs: mark desktop/ as the deprecated Buzz 0.X app (#8840)

## Summary

`desktop/` is the Buzz 0.X desktop app. Buzz 1.0 lives in
[block/buzz-app](https://github.com/block/buzz-app), and its README
already says so. Nothing in this repo says it, so contributors and
agents keep opening feature PRs against `desktop/`.

This PR adds the rule in three places:

- **`AGENTS.md`**: a notice at the top of "Desktop App" and a note in
the repo layout. Agents read this file first. The notice also says that
CLI PRs do not update the 0.X agent guidance in `nest_skill.md`.
- **`CONTRIBUTING.md`**: a new item in "PRs We're Unlikely to Merge".
- **`desktop/README.md`**: a notice at the top.

The rule: build desktop features in block/buzz-app, or as a Buzz plugin.
Change `desktop/` here only to fix a 0.X bug, or to keep it compiling
after a shared crate changes.

## Testing

- This PR changes only documentation. I checked the diff and the two
block/buzz-app links.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Signed-off-by: Blake McAnally <bmcanally@squareup.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

AGENTS.md

@@ -74,11 +74,12 @@ confirmation, not app runs.
 
 ## Ecosystem
 
-Buzz spans five repos. This one (`block/buzz`) is the OSS source for the relay, desktop, mobile, and CLI. The others handle internal builds and deployment:
+Buzz spans six repos. This one (`block/buzz`) is the OSS source for the relay, mobile, CLI, and the deprecated Buzz 0.X desktop app. [block/buzz-app](https://github.com/block/buzz-app) is the OSS source for the Buzz 1.0 desktop and web app. The others handle internal builds and deployment:
 
 | Repo | Purpose |
 |------|---------|
-| [block/buzz](https://github.com/block/buzz) | OSS source — relay, desktop app, mobile app, CLI, agent harness |
+| [block/buzz](https://github.com/block/buzz) | OSS source — relay, mobile app, CLI, agent harness, deprecated Buzz 0.X desktop app |
+| [block/buzz-app](https://github.com/block/buzz-app) | OSS source — Buzz 1.0 desktop and web app; all new desktop feature work |
 | [squareup/buzz-releases](https://github.com/squareup/buzz-releases) | Buildkite pipelines producing Block-signed macOS + iOS builds with `-block` desktop version suffix |
 | [squareup/sprout-oss](https://github.com/squareup/sprout-oss) | CI pipeline building the relay Docker image and pushing to internal ECR |
 | [squareup/block-coder-tf-stacks](https://github.com/squareup/block-coder-tf-stacks) | Terraform + ArgoCD deploying the relay to the staging Kubernetes cluster |
@@ -130,7 +131,7 @@ crates/
   buzz-test-client    # Integration test client and E2E test suite
   sprig               # All-in-one harness bundling ACP, agent, and dev MCP
 
-desktop/              # Legacy Tauri 2 + React 19 desktop app (being replaced by block/buzz-app)
+desktop/              # Buzz 0.X Tauri desktop app (deprecated: new desktop work goes to block/buzz-app)
 web/                  # Browser web client (repo browser, served by the relay)
 mobile/               # Flutter mobile app
 migrations/           # SQL migrations (run at relay startup when BUZZ_AUTO_MIGRATE is on: binary default off, Helm default on; deployments at scale apply them out of band — see docs/database.md)
@@ -646,9 +647,15 @@ description. See [PR #803](https://github.com/block/buzz/pull/803).
 
 ## Desktop App
 
-> **Legacy.** `desktop/` will soon be superseded by
-> [block/buzz-app](https://github.com/block/buzz-app). Never use it as an
-> implementation or design reference for the mobile app, which is current.
+> **`desktop/` is deprecated.** It is the Buzz 0.X desktop app. The Buzz 1.0
+> desktop app lives in [block/buzz-app](https://github.com/block/buzz-app).
+> Do not open block/buzz PRs that add features to `desktop/`; build desktop
+> features in block/buzz-app (or as a Buzz plugin) instead. Change `desktop/`
+> here only to fix a 0.X bug, or to keep it compiling when a shared crate it
+> depends on changes. This includes agent guidance bundled with the 0.X app,
+> such as `desktop/src-tauri/src/managed_agents/nest_skill.md`: CLI PRs do not
+> need to update it. Never use it as an implementation or design reference for
+> the mobile app, which is current.
 
 The desktop app is Tauri 2 + React 19 + Vite + Tailwind CSS. Features are
 organized under `desktop/src/features/`. Biome handles linting and formatting.

CONTRIBUTING.md

@@ -442,6 +442,11 @@ because we can't safely review them without prior discussion:
 - **Cosmetic renames or style-only churn** that doesn't fix a bug or improve
   clarity
 - **Entirely new features** with no prior discussion
+- **Feature work in `desktop/`** — `desktop/` is the deprecated Buzz 0.X
+  desktop app. Build desktop features in
+  [block/buzz-app](https://github.com/block/buzz-app) instead; change
+  `desktop/` here only to fix a 0.X bug or to keep it compiling after a shared
+  crate change
 - **Drive-by changes bundled into an unrelated fix** — split them out
 
 If you're considering any of these, open an issue first and we'll tell you
@@ -482,15 +487,18 @@ to existing clients.
 ## Ecosystem
 
 Buzz is developed across multiple repositories. This repo (`block/buzz`)
-is the open-source home for all application code — the relay, desktop app,
-mobile app, CLI, and agent harness. Internal repositories handle
+is the open-source home for the relay, mobile app, CLI, and agent harness.
+It also holds `desktop/`, the deprecated Buzz 0.X desktop app. The Buzz 1.0
+desktop and web app lives in [block/buzz-app](https://github.com/block/buzz-app),
+and all new desktop feature work goes there. Internal repositories handle
 enterprise-signed builds and infrastructure deployment.
 
 See [AGENTS.md § Ecosystem](AGENTS.md#ecosystem) for the full repo table and
 dependency diagram.
 
-**External contributors:** Fork `block/buzz`, open a PR, and CI runs
-automatically. No special access is required.
+**External contributors:** Fork `block/buzz` (or `block/buzz-app` for
+desktop work), open a PR, and CI runs automatically. No special access is
+required.
 
 **Block team members:** See the internal
 [sprout-releases CONTRIBUTING.md](https://github.com/squareup/sprout-releases/blob/main/CONTRIBUTING.md)

desktop/README.md

@@ -1,5 +1,11 @@
 # Buzz
 
+> [!IMPORTANT]
+> **This is the deprecated Buzz 0.X desktop app.** Buzz 1.0 lives in
+> [block/buzz-app](https://github.com/block/buzz-app). Build new desktop
+> features there. Change this directory only to fix a 0.X bug, or to keep it
+> compiling after a shared crate change.
+
 Desktop chat shell with:
 
 - Tauri + React + TypeScript + Vite