docs: add Gradle 9 developer skill for monorepo builds

[jamesfredley]

Summary

Adds a Gradle 9 developer skill so agents write Gradle the way this monorepo already does on 8.0.x, instead of inventing generic Gradle patterns that burn review cycles.

The skill is intentionally repo-first: when official Gradle docs and this monorepo disagree, this repo wins (configuration cache off, no version catalogs, native platform() instead of Spring DM for real apps, heavy projectDir remapping, presence-based -P flags, custom BOM validator).

Changes

1. .agents/skills/gradle-developer/SKILL.md - monorepo Gradle guide covering:

   • Multi-build topology (build-logic, grails-gradle, grails-forge, end-to-end, gradle-bootstrap)
   • Numbered change workflow (siblings → settings path → scoped verify)
   • BOM / platform() dependency management and validateDependencyVersions (direction-aware fixes; AGENTS.md rule 14)
   • Convention plugin IDs (fully qualified) + grails-publish provenance note
   • Wrapper sync via gradle-bootstrap (copies to forge/grails-gradle/end-to-end, moves into root) + remaining manual locations
   • end-to-end 3-step flow (build/local-maven / TestCaseMavenRepo; no includeBuild of root)
   • Build cacheability from production PRs (Improve Gradle build cacheability #15483, Fix overlapping task outputs preventing build cache usage #15532, Fix build cache hits for FindMainClassTask and GroovyPageForkCompileTask #15537): portable Test props, no overlapping outputs, SBOM classpath normalization, upToDateWhen traps, Develocity FROM-CACHE verification
   • base.dir dual rule: required @Internal compile provider (daemon isolation / grails.factories) vs forbidden absolute Test system properties
   • Configuration hygiene from recent PRs (fix: defer compile-classpath probes out of GroovyCompile task configuration #16076, Fix afterEvaluate race between applyGrailsBom and GrailsCliGradlePlugin #16009, Declare processResources ReplaceTokens values as task inputs #16006, feat: Replace Spring Dependency Management plugin with Gradle platform + lightweight BOM property overrides #15467, build(deps): update Gradle to 9.6.0 #15763, …)
   • Spring DM exception: only the intentional regression fixture at grails-test-examples/spring-dependency-management
   • Agent quality bar (Problem / Fix / Why / Verification evidence)
   • Anti-patterns + Definition of Done including cacheability gates

2. AGENTS.md

   • Mandatory skill bullet + skills table entry for gradle-developer
   • mono-repo-integration table entry
   • Spring Boot BOM pin corrected to 4.1.x (matches Technology Stack)
   • build-logic uses its own wrapper (cd build-logic && ./gradlew build)

Research basis

Polished against:

Source	What we took
#15483 (user-linked)	Cacheability: no absolute base.dir in Test props; SBOM normalization; doFirst vs cache eligibility
#15532 / #15537	Overlapping outputs kill caching; bad upToDateWhen blocks cache load; per-task paths
#16076 / #16009 / #16006	No config-time classpath resolve; afterEvaluate races; ReplaceTokens as task inputs
#15467 / #15730	Platforms replace Spring DM; prefer Boot BOM inheritance
#15763 / gradle-bootstrap	Multi-location wrapper discipline (root moved by bootstrap)
#16069 / #16078	Cheap functional substitution; CLI/test tier off production classpath
High-quality 8.0.x AI PRs (#15798 skill, #15797 style)	Problem/Fix/Why/Verification + scoped evidence
Live tree fact-check	Plugin IDs, end-to-end README, SbomPlugin normalization, BaseDirArgumentProvider

Also skimmed recent merged 8.0.x and 7.0.x Gradle/build/agent PRs so the skill encodes durable patterns, not one-off version bumps.

Notes

• Docs/skills only; no build or runtime code changes.
• Based on current 8.0.x Gradle 9.6.0 layout.
• Goal: AI can write monorepo-acceptable Gradle on the first try.

Test plan

• Spot-check skill claims against settings.gradle, dependencies.gradle, end-to-end/README.md, gradle-bootstrap/build.gradle, SbomPlugin, BaseDirArgumentProvider, convention plugins
• Dual review-gate (oracle + codex) on polish commit
• git diff --check
• Human skim of skill for anything still wrong or missing for day-to-day Gradle work

[Copilot]

Pull request overview

Adds repository-specific Gradle 9 build guidance as an agent “skill”, and wires it into AGENTS.md so contributors/agents are prompted to load the Gradle guidance before touching build logic. This helps keep Gradle changes aligned with established 8.0.x monorepo conventions (composite builds, BOM/platform usage, wrapper sync, and Gradle 9 configuration-time pitfalls).

Changes:

• Introduces .agents/skills/gradle-developer/SKILL.md with monorepo-first Gradle 9.6.x guidance (topology, BOM rules, wrapper sync, and configuration hygiene).
• Updates AGENTS.md to explicitly include Gradle build changes in the “mandatory skill” list and adds the new skill to the skills table.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
AGENTS.md	Adds Gradle-specific skill guidance to the mandatory skills list and table so build changes are routed to the new skill.
.agents/skills/gradle-developer/SKILL.md	New Gradle 9 monorepo build skill documenting established repo patterns (composite builds, BOM/platform rules, wrapper sync, and Gradle 9 configuration hygiene).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.3462%. Comparing base (2e2ce06) to head (e4d26c5).
⚠️ Report is 174 commits behind head on 8.0.x.

Additional details and impacted files

@@                Coverage Diff                 @@
##                8.0.x     #16083        +/-   ##
==================================================
+ Coverage     51.8811%   52.3462%   +0.4651%     
- Complexity      18118      18298       +180     
==================================================
  Files            2046       2036        -10     
  Lines           96274      96347        +73     
  Branches        16727      16829       +102     
==================================================
+ Hits            49948      50434       +486     
+ Misses          38954      38489       -465     
- Partials         7372       7424        +52     

see 61 files with indirect coverage changes

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