Generate the detekt config in a task so it survives configuration cache reuse

[rasros]

What changed

• The lint plugin no longer writes build/tmp/eignex-detekt.yml eagerly at configuration time. A registered task (writeEignexDetektConfig) with a declared input (the content) and output (the file) materializes it instead, and the detekt and baseline tasks depend on it.
• Added a TestKit regression test that runs detekt with the configuration cache, clears the build dir, and runs again, asserting the second run reuses the cache and still succeeds.

Why

The config was written as a side effect during project configuration, and the detekt tasks just pointed their --config at it. When the configuration cache is reused the configuration phase is skipped, so the write never happens. On a fresh checkout that restored only the Gradle home and not build/ (a CI runner with gradle/actions/setup-gradle and a cache-encryption-key), every detekt task then failed with "Path '.../build/tmp/eignex-detekt.yml' passed to --config does not exist". Making generation a task puts it in the task graph, where it runs on every build including configuration-cache hits, and makes the output up-to-date-checkable and cacheable.

Testing

Ran the lint plugin TestKit suite (5 tests) green. Confirmed the new test fails without the plugin change (reproduces the missing-config failure) and passes with it. Verified a negative sample still fires UnnecessaryFullyQualifiedName, and that a positive sample run twice with --configuration-cache reports "Configuration cache entry stored" then "Configuration cache entry reused".