[Good first issue]: Add a Kotlin API recipe for custom tool capabilities

[morning-verlu]

Goal

Add a short recipe to docs/KOTLIN_API.md that shows how a Kotlin/JVM user can define or wire a custom tool behind a ToolCapabilityGrant.

Why this helps

KAI OS is trying to be legible as an embeddable Agent OS runtime, not only a CLI. A small recipe makes the syscall/capability model easier for Kotlin developers to understand.

Suggested scope

• Add a compact section to docs/KOTLIN_API.md.
• Keep the example deterministic and no-key.
• Link to examples/kotlin-runtime-api if the recipe builds on the existing example.
• Avoid adding a new provider or network dependency.

Acceptance

• docs/KOTLIN_API.md explains the custom-tool/capability path in under one screen.
• Existing commands still pass:

./scripts/repository-ci-smoke.sh

A docs-only PR is welcome.

[morning-verlu]

Maintainer guidance for whoever picks this up:

A good PR can stay docs-only. Please keep it small and focused on the custom-tool capability path.

Suggested shape:

1. Add a section to docs/KOTLIN_API.md titled something like Custom Tool Capability Recipe.
2. Show a compact Kotlin snippet that wires a tool behind an explicit capability grant.
3. Explain in 2-3 bullets why this is treated as a syscall boundary, not arbitrary model access.
4. Link to examples/kotlin-runtime-api for the runnable full example.

Useful verification for this issue:

git diff --check
./gradlew -p examples/kotlin-runtime-api run

If you only change docs and cannot run Gradle locally, include that clearly in the PR notes and paste git diff --check output.