Previously:
- The bash script required specifying `SKIA_VERSION` separately from
`gradle.properties`, leading to version mismatches
- Script hardcoded `SKIA_TARGET` default to `iosSim`, requiring manual
override on non-macOS systems

This change:
- Added `skiaVersionFromEnvOrProperties` property and `printSkiaVersion`
task. The bash script now queries Gradle for the version instead of
maintaining its own default.
- Target platform now defaults to current OS (`hostOs.id`) instead of
hardcoded `iosSim`.
- Introduced `BuildLocalSkiaTask` that handles Python build script
invocation, architecture detection, and validation.
- Added `SkiaTarget` enum centralizing platform-specific Gradle flags
and architecture mappings.
- Simplified bash script: Now focuses solely on git operations
(clone/checkout skia-pack), delegating all build logic to Gradle.

### Usage
```sh
# Version from gradle.properties, target auto-detected from OS
./build-with-local-skia.sh

# Override either via environment variables
SKIA_VERSION=m138-80d088a-2 SKIA_TARGET=iosSim ./build-with-local-skia.sh

# Or use Gradle tasks directly
./gradlew prepareLocalSkiaBuild -Pskia.target=linux
```