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>

BUILDING.md

@@ -0,0 +1,24 @@
+# Building Scarf
+
+Scarf is a native macOS app built with Xcode. For contributor builds, use the local script:
+
+```bash
+./scripts/local-build.sh
+```
+
+Requirements:
+
+- macOS 26.2 or newer (the app's deployment target)
+- Xcode 26.2 or newer, selected by `xcode-select`
+- Metal toolchain installed
+- Hermes installed at `~/.hermes/` (see the project README for setup)
+
+If the Metal toolchain is missing, the script will offer to install it in interactive shells. You can also install it manually:
+
+```bash
+xcodebuild -downloadComponent MetalToolchain
+```
+
+`scripts/local-build.sh` resolves Swift package dependencies, detects `arm64` vs `x86_64`, and builds the Debug app unsigned. Signing is intentionally disabled for local Debug builds so contributors do not need the maintainer's Apple Developer account.
+
+Release signing is separate from contributor builds. Maintainers should continue using the existing release process for signed distributable builds.