feat: agent run loop, Generate[T], reflect-derived schemas

Phase 5:
- agent/: model + system prompt + toolboxes composition; bounded
  tool-dispatch loop (default 10 steps); panic-proof tool execution;
  unknown-tool and duplicate-name handling; history continuation; step
  observers; partial results on ErrMaxSteps/errors (ADR-0012)
- llm.SchemaFor[T]: strict-compatible JSON schemas from Go types
  (nullable pointers, description/enum tags, recursion rejected)
- majordomo.Generate[T]: typed structured output with fence-stripping
  decode and model-naming errors
- README agents/structured-output sections + matrix synced

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/adr/README.md

@@ -15,3 +15,4 @@ One decision per file, append-only; supersede rather than rewrite.
 | [0009](0009-multimodal-strategy.md) | Multimodal strategy — normalize per target, enforce at provider | Accepted |
 | [0010](0010-tools-structured-output-mapping.md) | Tools and structured output — canonical shape, native mappings | Accepted |
 | [0011](0011-google-provider.md) | Google provider on the official Gen AI SDK | Accepted |
+| [0012](0012-agent-loop.md) | Agent run loop | Accepted |