feat(build): contributor-friendly local-build.sh + BUILDING.md

Adds `scripts/local-build.sh` for unsigned command-line Debug builds
so contributors without an Apple Developer account can clone, build,
and run without provisioning gymnastics. The script:

- Detects arm64 / x86_64
- Verifies xcode-select, xcrun, xcodebuild are present
- Probes the Metal toolchain and offers an interactive install (gated
  on `[[ -t 0 && -z "${CI:-}" ]]` — CI never gets prompted)
- Resolves Swift packages, builds Debug with signing disabled
- Optionally `ditto`s the result to /Applications/scarf.app on
  explicit y/N

`BUILDING.md` documents prerequisites alongside the script. Existing
canonical Release universal CLI in README stays — `local-build.sh`
is an alternative for contributors, not a replacement for the
shipping build.

Cherry-picked from #76 with thanks to @unixwzrd. BUILDING.md's
prerequisites are corrected to match the actual deployment target
(macOS 26.2, Xcode 26.2+).

Co-Authored-By: M S <unixwzrd.register@mac.com>
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CONTRIBUTING.md

@@ -8,6 +8,8 @@ Thanks for your interest in contributing to Scarf.
 2. Open `scarf/scarf.xcodeproj` in Xcode 26.3+
 3. Build and run (requires macOS 26.2+ and Hermes installed at `~/.hermes/`)
 
+For an unsigned command-line Debug build without an Apple Developer account, run [`./scripts/local-build.sh`](scripts/local-build.sh). See [BUILDING.md](BUILDING.md) for prerequisites.
+
 ## Architecture
 
 Scarf uses the MVVM-Feature pattern. Each feature is a self-contained module under `Features/`: