Add schema-aware custom-agent outputs

[DavidBakerEffendi]

Description

Adds an explicit callCustomAgentWithSchema path for schema-constrained child agents while preserving legacy callCustomAgent behavior. The child agent validates schemas, runs its normal tool loop, and makes one final structured-answer call when a schema is supplied.

The schema-aware tool no longer depends on the parent model copying a large schema object exactly. It accepts a full schema object, a schema name string, or a partial {name, schema:{}} reference, then resolves the full validated schema from the parent task text before dispatch.

Key Changes:

• Added schema-aware custom-agent execution and parallel handling so each child can return one structured result without falling back to Markdown.
• Wired schema validation and structured final-answer calls through CustomAgentExecutor, CustomAgentTools, and ParallelCustomAgent.
• Resolved schema references server-side for brittle parent-model tool calls, including schema-name and empty-schema-object references.
• Gated callCustomAgentWithSchema exposure on model JSON-schema support and kept schema-aware child failures as hard failures.
• Let SearchAgent run read-only custom-agent batches through ParallelCustomAgent, including schema-aware specialist flows.
• Expanded test coverage for final structured output, invalid schema rejection, unsupported-model failures, schema-reference resolution, and parallel child execution.

Touch Points:

• app/src/main/java/ai/brokk/agents/ArchitectAgent.java
• app/src/main/java/ai/brokk/agents/LutzAgent.java
• app/src/main/java/ai/brokk/agents/SearchAgent.java
• app/src/main/java/ai/brokk/executor/agents/CustomAgentExecutor.java
• app/src/main/java/ai/brokk/executor/agents/CustomAgentResponseSchemaResolver.java
• app/src/main/java/ai/brokk/executor/agents/CustomAgentTools.java
• app/src/main/java/ai/brokk/executor/agents/ParallelCustomAgent.java
• app/src/main/java/ai/brokk/mcpserver/BrokkExternalMcpServer.java
• app/src/test/java/ai/brokk/agents/SearchAgentTest.java
• app/src/test/java/ai/brokk/executor/agents/CustomAgentExecutorTest.java
• app/src/test/java/ai/brokk/executor/agents/ParallelCustomAgentTest.java

Summary

• Introduces an explicit schema-aware custom-agent path for SlopCop-style batched specialist flows.
• Keeps the existing Markdown/string callCustomAgent path unchanged.
• Treats schema failures and unsupported schema-capable models as hard failures for the child execution path.
• Avoids brittle large-schema copying by resolving schema references inside Brokk before child execution.

Testing

• ./gradlew :app:test --tests ai.brokk.executor.agents.ParallelCustomAgentTest --tests ai.brokk.executor.agents.CustomAgentExecutorTest --tests ai.brokk.agents.SearchAgentTest --tests ai.brokk.agents.LutzAgentTest
• ./gradlew fix tidy
• ./gradlew analyze