ci: pin Go toolchain to 1.24.9 across all workflows (#6)

CI workflows used inconsistent floating Go versions (ci.yml ^1.24;
build/preview/release ^1.25) while libcore/go.mod declares 'go 1.24.7'
and 'toolchain go1.24.9'. Pin all setup-go steps to the exact 1.24.9 to
match go.mod for reproducible, consistent native-core builds.

This is a reproducibility alignment only: no go.mod, source, or dependency
changes, and no Go language-version bump.

Author: hawkff

.github/workflows/build.yml

@@ -28,7 +28,7 @@ jobs:
         if: steps.cache.outputs.cache-hit != 'true'
         uses: actions/setup-go@v6
         with:
-          go-version: ^1.25
+          go-version: '1.24.9'
       - name: Native Build
         if: steps.cache.outputs.cache-hit != 'true'
         run: ./run lib core

.github/workflows/ci.yml

@@ -33,7 +33,7 @@ jobs:
         if: steps.cache.outputs.cache-hit != 'true'
         uses: actions/setup-go@v6
         with:
-          go-version: ^1.24
+          go-version: '1.24.9'
       - name: Native Build
         if: steps.cache.outputs.cache-hit != 'true'
         run: ./run lib core

.github/workflows/preview.yml

@@ -24,7 +24,7 @@ jobs:
         if: steps.cache.outputs.cache-hit != 'true'
         uses: actions/setup-go@v6
         with:
-          go-version: ^1.25
+          go-version: '1.24.9'
       - name: Native Build
         if: steps.cache.outputs.cache-hit != 'true'
         run: ./run lib core

.github/workflows/release.yml

@@ -30,7 +30,7 @@ jobs:
         if: steps.cache.outputs.cache-hit != 'true'
         uses: actions/setup-go@v6
         with:
-          go-version: ^1.25
+          go-version: '1.24.9'
       - name: Native Build
         if: steps.cache.outputs.cache-hit != 'true'
         run: ./run lib core