bug(dev): `just dev` leaves non-executable sidecar stubs, breaking ACP authentication

[kimparrot]

Summary

A source build launched with just dev can leave the desktop sidecar helpers as zero-byte, non-executable files. The desktop app then fails when it invokes buzz-acp for Claude Code sign-in discovery:

Couldn't load sign-in options: failed to run buzz-acp auth helper: Permission denied (os error 13)

This also prevents other desktop-managed agent features that require the bundled sidecars.

Reproduction

1. Clone the repository on macOS (Apple Silicon).
2. Run the documented setup, then start the app with just dev.
3. In the desktop app, open the Claude Code integration and attempt to load sign-in options.

The files under desktop/src-tauri/binaries/ are placeholders created by _ensure-sidecar-stubs and remain zero bytes / mode 0644.

Expected

just dev should make executable buzz-acp, buzz-agent, buzz-dev-mcp, buzz, and git-credential-nostr sidecars available to the Tauri desktop process.

Actual

The Tauri app attempts to execute the buzz-acp helper and macOS returns Permission denied (os error 13).

Likely cause

_ensure-sidecar-stubs intentionally creates placeholder binaries for Tauri compile-time validation, but the dev recipe does not copy the compiled helpers into desktop/src-tauri/binaries/ afterward. desktop-standalone already performs that copy, so the behavior is inconsistent.

A local workaround was to build the helper binaries and copy them into desktop/src-tauri/binaries/<name>-aarch64-apple-darwin; after that, buzz-acp auth-methods --json returned the Claude sign-in methods successfully.

Suggested fix

Reuse the copy-and-chmod +x block from desktop-standalone in just dev after its cargo build step, or factor it into a shared recipe used by both launch paths.