fix(desktop): give Windows agent children a usable PATH and start dev

[al3rez]

Two separate things stop Buzz working on Windows. Both are small, but together they make the desktop app unusable there.

Agents can't find node

Every managed agent dies the moment it starts. From %APPDATA%\xyz.block.buzz.app\agents\logs\:

'"node"' is not recognized as an internal or external command,
ERROR buzz_acp: agent initialize failed: Agent process exited unexpectedly
Error: all 24 agents failed to start — cannot continue

build_augmented_path assembles the child's PATH from Buzz's managed directories plus the user's login-shell PATH. On Windows the latter is always None — fetch_login_shell_path_inner returns early there, correctly, since Git Bash reports POSIX colon-delimited entries (/c/Users/...) that a native child can't use.

The comment says children "inherit the real Windows PATH instead." They don't. runtime.rs does command.env("PATH", path), which replaces the inherited value rather than extending it, so the agent ends up with Buzz's managed directories and nothing else. No node, no npm, no git. The npm .cmd shims fail on their first line.

This appends the process PATH on Windows when no shell PATH was supplied. It's gated on !parts.is_empty() so callers that pass no local context still get None back, matching the existing rule about not manufacturing a PATH out of ambient state.

tauri dev doesn't start

'exec' is not recognized as an internal or external command
The "beforeDevCommand" terminated with a non-zero status code.

beforeDevCommand is exec ./node_modules/.bin/vite. exec is a POSIX shell builtin, and Tauri runs the script through cmd.exe on Windows. Calling Vite through node works everywhere.

One caveat worth a review from someone on the macOS dev loop: exec was presumably there to avoid an intermediate shell process so signals reach Vite directly. This change gives that up on Unix. If that matters more than I think, the alternative is a platform-specific config file, which is uglier but keeps the current behaviour.

Testing

Windows 11 (26100). Two new unit tests cover the PATH change: one asserts the process PATH is appended after the managed directories, the other that no PATH is manufactured without local context. cargo test -p buzz-desktop --lib managed_agents::runtime::path passes, and the existing Unix tests are untouched.

For the dev command, pnpm exec tauri dev now starts cleanly with no platform-specific config override:

Running BeforeDevCommand (`node ./node_modules/vite/bin/vite.js`)
VITE v8.0.16  ready in 984 ms

Neither change affects macOS or Linux behaviour: the PATH addition is behind #[cfg(windows)], and on Unix shell_path is always Some, so the new branch can't be reached even if the cfg were removed.