Extract gradle-conventions + personal-stack adopt

[jorisjonkers-dev-agents]

Objective: publish Gradle convention plugins as Maven plugin-marker artifacts; personal-stack consumes via pluginManagement.

Notes: resolve version drift (gradle.properties Kotlin 2.1.20/Spring 4.0.4/jOOQ 3.19.18 vs build-logic 2.3.21/4.0.6/3.21.4). Make jOOQ/OpenAPI/detekt-path/toolchain opt-in. Add TestKit coverage. Risk: plugin resolution precedes build.

Work:

• Build out ExtraToast/gradle-conventions (plugins, stable IDs, publish)
• personal-stack replaces includeBuild build-logic with pinned plugin coords
• GitHub Packages creds in pluginManagement + dependencyResolutionManagement

Part of #605.

[jorisjonkers-dev-agents]

Extraction merged. ExtraToast/gradle-conventions#1 (built by a Codex-high agent, CI green: ./gradlew build+test + TestKit smoke test pass). Plugin IDs dev.extratoast.{kotlin,detekt,ktlint,spring,testing,test-logging,jooq-codegen}; PS-specifics opt-in via extensions; version drift resolved to build-logic values. REMAINING: cut v0.1.0 (release-please) -> publish to GitHub Packages, then personal-stack adoption (replace includeBuild build-logic with pinned plugin coords + libs.versions.toml + Renovate) — gradle-gated, verified by me/CI.