fix(desktop): discover bun-installed agent CLIs in ~/.bun/bin (#3343)

## Problem

`common_binary_paths()` probes mise shims, `~/.local/bin`, volta, asdf,
and (further down `resolve_command_uncached`) nvm's default bin dir —
but not bun's global bin directory, `~/.bun/bin`.

bun's installer appends its bin dir to `~/.zshrc` / `~/.bashrc`, which
are **interactive**-only. A login shell never sources them, so
`find_via_login_shell()` can't recover the path either. That's the same
failure mode already called out in this file for nvm:

```rust
// Check nvm's default Node.js bin directory — nvm initializes via
// ~/.zshrc (interactive) which is not loaded by a login shell, so
// `node`, `npm`, and npm-global shims installed there are otherwise
// invisible.
```

So for a GUI-launched desktop app, every rung of the resolution ladder
misses a bun-installed CLI:

1. workspace dev dirs — no
2. `command_looks_like_path` — no, presets use bare names
3. Buzz-managed npm/node dirs — no
4. current process PATH — launchd's minimal PATH on a Finder launch
5. `find_via_login_shell` — `.zshrc` not sourced
6. `common_binary_paths()` — **`~/.bun/bin` absent**
7. nvm default bin — no

This matters because bun is a common install route for the agent CLIs
Buzz targets. Kimi Code in particular ships as an npm package
(`@moonshot-ai/kimi-code`), so `bun add -g` puts it at `~/.bun/bin/kimi`
— exactly where discovery doesn't look.

## Reproduction

On macOS with `codex` and `kimi` installed via bun, launching Buzz from
Finder:

- Kimi Code shows **"CLI needed"**
- both CLIs run fine in an interactive terminal

Probing the way `find_via_login_shell` does, in a clean environment:

```console
$ env -i HOME=$HOME /bin/zsh -l -c 'command -v -- codex; command -v -- kimi'
(nothing)
```

Launching the app with the bun dir on PATH resolves both immediately:

```console
$ env PATH="$HOME/.bun/bin:$PATH" /Applications/Buzz.app/Contents/MacOS/buzz-desktop
```

## Change

One entry appended to the home-relative list in `common_binary_paths()`.
It goes **last** so it cannot shadow a directory that already resolves —
the change can only add resolutions, never alter existing ones.

## Testing

`cargo fmt --check` passes.

I was not able to run the full `just ci` gate locally: `ring 0.17.14`
fails to build in this environment against the macOS 26.2 SDK (`cc`
error compiling `p256-nistz.c`), which is unrelated to this change.
Relying on CI for the rest — the diff adds one `PathBuf` to an existing
`Vec<PathBuf>` and introduces no new API.

## Notes

- Related to #3084, which adds `~/.kimi-code/bin` for the same class of
GUI-launch discovery failure. That covers Kimi's standalone installer;
this covers the bun/npm-global install route. They're complementary —
I've left a note on that PR.
- Only `~/.bun/bin` is added. bun's global packages live under
`~/.bun/install/global/node_modules` but are symlinked into
`~/.bun/bin`, so the single directory is sufficient.
- Worth noting `~/.bun/bin` contains no `node`/`npm`/`npx`, so appending
it can't shadow a system Node toolchain.

Signed-off-by: Xule Lin <43122877+linxule@users.noreply.github.com>

desktop/src-tauri/src/managed_agents/discovery.rs

@@ -19,7 +19,6 @@ const CLAUDE_CODE_AVATAR_URL: &str = "https://anthropic.gallerycdn.vsassets.io/e
 const CODEX_AVATAR_URL: &str = "https://openai.gallerycdn.vsassets.io/extensions/openai/chatgpt/26.5313.41514/1773706730621/Microsoft.VisualStudio.Services.Icons.Default";
 const BUZZ_AGENT_AVATAR_URL: &str =
     "https://raw.githubusercontent.com/block/buzz/refs/heads/main/crates/buzz-agent/buzz-agent.png";
-
 fn common_binary_paths() -> &'static [PathBuf] {
     static PATHS: OnceLock<Vec<PathBuf>> = OnceLock::new();
     PATHS.get_or_init(|| {
@@ -41,6 +40,7 @@ fn common_binary_paths() -> &'static [PathBuf] {
                 home.join(".local/bin"),
                 home.join(".volta/bin"),
                 home.join(".asdf/shims"),
+                home.join(".bun/bin"),
             ]);
         }
         // Windows well-known dirs for npm global shims and standalone installer targets.