fix(justfile): complete buzz rename in dev and staging recipes

[tellaho]

Overview

Category: fix
User Impact: just dev and just staging work again on a fresh checkout — they currently crash before reaching tauri dev, and staging would otherwise silently connect to the wrong relay.
Problem: #958 (the buzz backend rename) renamed the cargo crates and _ensure-sidecar-stubs to buzz-*, but three other parts of the build chain were left referencing the old sprout-* names, plus a separate env-var rename in scripts/instance-env.sh was not propagated:

1. The dev and staging recipes still reference SPROUT_TAURI_CONFIG / SPROUT_VITE_PORT / SPROUT_RELAY_URL, but scripts/instance-env.sh exports those as BUZZ_*. The recipes call JSON.parse(process.env.SPROUT_TAURI_CONFIG) on undefined and abort with SyntaxError: "undefined" is not valid JSON.
2. desktop/src-tauri/tauri.conf.json declares externalBin entries as binaries/sprout-{acp,agent,dev-mcp,...} and the staging + desktop-release-build recipes touch/cp sprout-* paths, but the stubs created by _ensure-sidecar-stubs are now buzz-*. tauri-cli aborts with resource path 'binaries/sprout-acp-aarch64-apple-darwin' doesn't exist.
3. The desktop shell itself still read SPROUT_RELAY_URL / SPROUT_RELAY_HTTP (relay.rs, build.rs). With the recipes exporting BUZZ_RELAY_URL, just staging would launch but the app would silently fall back to ws://localhost:3000 while the sidecar crates (which already read BUZZ_RELAY_URL) pointed at staging — a split-brain.
   Solution: Three narrow sweeps that finish refactor: rename sprout backend to buzz #958's direction:
4. SPROUT_TAURI_CONFIG / SPROUT_VITE_PORT / SPROUT_RELAY_URL → BUZZ_* in the dev and staging recipes so they read what instance-env.sh actually exports.
5. binaries/sprout-* → binaries/buzz-* in tauri.conf.json externalBin, the staging recipe's cp target, and the desktop-release-build stub touch calls — so all four sites agree on the buzz-* names that _ensure-sidecar-stubs already produces.
6. SPROUT_RELAY_URL / SPROUT_RELAY_HTTP → BUZZ_* in the desktop shell's read path (relay.rs:22,67 and the compile-time BUZZ_DESKTOP_BUILD_RELAY_{URL,HTTP} bake in build.rs) so the shell honors the same relay env vars as everything else.

Out of scope (pre-existing #958 gaps, follow-up): the managed-agent spawn handoff still uses SPROUT_* names (runtime.rs sets SPROUT_RELAY_URL/SPROUT_PRIVATE_KEY/SPROUT_ACP_*, but buzz-acp reads BUZZ_*; DEFAULT_ACP_COMMAND is still sprout-acp; KNOWN_AGENT_BINARIES only lists sprout-*). That handoff needs its own coordinated rename.

File changes

justfile
Renamed env-var references inside the dev and staging recipes from SPROUT_* to BUZZ_* to match scripts/instance-env.sh. Renamed the cp target in staging and the touch paths in desktop-release-build from binaries/sprout-* to binaries/buzz-* to match _ensure-sidecar-stubs.

desktop/src-tauri/tauri.conf.json
Renamed externalBin entries from binaries/sprout-{acp,agent,dev-mcp} and binaries/sprout to their binaries/buzz-* equivalents so they match the stubs produced by _ensure-sidecar-stubs and the binaries cargo emits.

desktop/src-tauri/src/relay.rs, desktop/src-tauri/build.rs
The desktop shell now reads BUZZ_RELAY_URL / BUZZ_RELAY_HTTP (and bakes BUZZ_DESKTOP_BUILD_RELAY_{URL,HTTP} at compile time) instead of the SPROUT_* names, matching the recipes and the renamed crates.

Reproduction Steps

1. Fresh checkout of main (or fresh worktree from main).
2. Run just staging (or just dev). Without this fix: SyntaxError: "undefined" is not valid JSON right after the cargo build, then later resource path 'binaries/sprout-acp-...' doesn't exist; even with those patched, the app would connect to ws://localhost:3000 instead of the staging relay. With this fix, the recipe proceeds into tauri dev and the shell honors BUZZ_RELAY_URL.