fix(tauri-build): honor CARGO_BUILD_TARGET when locating skill-daemon

[FourthWiz]

Summary

scripts/tauri-build.js locates the freshly built skill-daemon binary using only the explicit --target flag (explicitTarget, populated at :279-286); the CARGO_BUILD_TARGET environment variable has zero hits in the script. But cargo — invoked at :966 with the inherited env — does read CARGO_BUILD_TARGET and writes the binary under the target-triple directory. The repo's own .envrc:11-12 exports CARGO_BUILD_TARGET=aarch64-apple-darwin on macOS (and .envrc:2 says to direnv allow once after cloning), so on the documented dev setup the locator misses on every npm run tauri dev.

This PR falls back to process.env.CARGO_BUILD_TARGET before the empty triple:

const triple = explicitTarget || process.env.CARGO_BUILD_TARGET || "";

What actually breaks (and what doesn't)

Being upfront: the daemon still starts either way — resolve_daemon_bin_path() (src-tauri/src/daemon_cmds.rs:126-183) papers over the miss via its current_exe() sibling probe, and its relative candidate list even hardcodes the triple path. So this is not "the daemon is broken." What the miss does cause, on every direnv-enabled dev run:

1. A spurious ⚠ skill-daemon binary not found after build — Tauri will attempt auto-launch warning — on the repo's own documented dev setup.
2. killPortOwner(daemonPort) is skipped, so a stale dev daemon on the dev port is not reaped before the new run.
3. The SKILL_DAEMON_BIN handoff is skipped (redundant in practice — see above — but skipped nonetheless).

Before/after (macOS, Apple Silicon; command: CARGO_BUILD_TARGET=aarch64-apple-darwin npm run tauri dev)

Before:

🔧 Building skill-daemon…
    Finished `dev` profile [unoptimized] target(s) in 1.42s
⚠ skill-daemon binary not found after build — Tauri will attempt auto-launch

(cargo had just written src-tauri/target/aarch64-apple-darwin/debug/skill-daemon; later in the same run the app auto-launched it anyway — the warning is spurious.)

After:

🔧 Building skill-daemon…
    Finished `dev` profile [unoptimized] target(s) in 1.51s
🚀 Starting daemon: …/src-tauri/target/aarch64-apple-darwin/debug/skill-daemon
INFO skill daemon listening addr=127.0.0.1:18445

Zero occurrences of the warning in the after-run log.

Scope note

The same explicitTarget || "" pattern occurs twice; this PR fixes both:

• :970 — the skill-daemon locator (verified with the before/after transcript above);
• :1091 — the macOS .app bundle lookup for widget embedding in the build subcommand (same pattern applied for consistency; the existing no-triple candidate remains as fallback, so behavior without CARGO_BUILD_TARGET is unchanged; not separately transcript-verified).

Testing

No vitest coverage exists for scripts/tauri-build.js, and a test harness for a dev script would exceed this fix's blast radius — the before/after transcripts above are the verification.