feat: TaskDefinition syntactic sugar — higher-level API compiling to Worker + Binding

[mdproctor]

Part of epic #24.

Port and adapt TaskDefinition from casehub-poc's casehub-core as a higher-level authoring API.

What it does

TaskDefinition lets users define case behaviour (entry criteria, produced keys, execution logic) without constructing Worker + Binding pairs directly against the raw engine model. The framework compiles a TaskDefinition into the corresponding engine primitives at registration time.

POC reference

io.casehub.core.TaskDefinition — interface with entryCriteria(), producedKeys(), execute(CaseFile), timeout, and retry policy.
io.casehub.core.TaskDefinitionRegistry — registration with circular dependency validation.

Scope

• Decide whether TaskDefinition belongs in api/ (shared across repos) or is app-local to this service
• Port the interface, adapting CaseFile references to the current engine model (CaseContext)
• Implement compilation: TaskDefinition → Worker + Binding at registration time
• Port circular dependency validation from TaskDefinitionRegistry

Open question

Whether entryCriteria / producedKeys semantics map cleanly to the current engine's Worker + Binding model, or need adaptation.

Depends on

• feat: CaseDefinitionRegistry shim — YAML and Java-lambda case definition coexistence #27 (registry shim) — TaskDefinition-compiled workers need to coexist with YAML definitions