feat(gradle): release v1.6.0 — Gradle build file version bump

Add internal/gradle package with ReadVersion and WriteVersion for
build.gradle (Groovy DSL, single-quoted) and build.gradle.kts (Kotlin
DSL, double-quoted). Quote style is preserved on write. regexp.QuoteMeta
ensures version strings with dots or special characters are safe.

Config follows the established multi-value pattern:
- gradle.build_file: single path (opt-in, no default)
- gradle.build_files: list for multi-module projects (overrides build_file)
- --gradle flag overrides build_file and clears build_files

100% per-package statement coverage maintained across all 13 packages;
FuzzReadVersion and FuzzWriteVersion added per fuzzing guidelines.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/content/configuration.md

@@ -45,6 +45,14 @@ node:                                                # opt-in — omit section t
   #   - "packages/frontend/package.json"
   #   - "packages/backend/package.json"
 
+gradle:                                              # opt-in — omit section to skip
+  # build_file: "build.gradle"                       # Groovy or Kotlin DSL; single path
+
+  # Multi-module: list overrides build_file.
+  # build_files:
+  #   - "build.gradle"
+  #   - "module-a/build.gradle"
+
 gitlab:
   url: "https://gitlab.example.com"                  # or env CI_SERVER_URL
   token: ""                                          # prefer env GITLAB_TOKEN
@@ -119,3 +127,24 @@ node:
     - "packages/frontend/package.json"
     - "packages/backend/package.json"
 ```
+
+## Gradle support
+
+The `gradle` section is opt-in — if omitted, no build file is touched. Both Groovy DSL (`version = '1.2.3'`) and Kotlin DSL (`version = "1.2.3"`) are supported; the original quote style is preserved on write.
+
+```yaml
+gradle:
+  build_file: "build.gradle"
+```
+
+Use `build_files` for multi-module projects:
+
+```yaml
+gradle:
+  build_files:
+    - "build.gradle"
+    - "module-a/build.gradle"
+    - "module-b/build.gradle"
+```
+
+The `--gradle <path>` CLI flag sets a single build file path and clears `build_files`.