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>

progress.md

@@ -1,5 +1,23 @@
 # progress
 
+## 2026-06-10 — Phase 5: agent loop, Generate[T], schema derivation
+
+**Landed:** `agent/` (ADR-0012): New(model, system, opts) with toolboxes,
+max steps (default 10), per-step request options, agent-level observers +
+per-run OnStep, WithHistory continuation (Result.Messages round-trips),
+sequential tool dispatch through panic-recovering ExecuteTool, unknown
+tools → IsError results, duplicate tool names fail loudly, partial Result
+preserved on ErrMaxSteps/model errors/cancellation. The agent.Skill
+interface ships here (instructions + tools composition is tested with a
+stub); the skill package with real implementations is Phase 6.
+`llm.SchemaFor[T]` reflect-derived strict-compatible JSON schemas
+(pointers→nullable anyOf, description/enum tags, maps/slices/time/RawMessage,
+recursion rejected) and root `majordomo.Generate[T]` (schema injection,
+fence-stripping decode, model-naming errors). 15 agent tests + schema +
+Generate suites, all hermetic.
+
+**Next:** Phase 6 — skill package + two example skills.
+
 ## 2026-06-10 — Phase 4: Google provider (official genai SDK)
 
 **Landed:** `provider/google` on google.golang.org/genai v1.59.0 (ADR-0011):