feat: skills — additive instruction+tool bundles, clock + calc examples

Phase 6: skill.New constructor satisfying the agent.Skill contract;
instruction-only skills; ordered additive composition; skill/clock
(injectable-clock time tools) and skill/calc (recursive-descent arithmetic
evaluator) as ready-made examples with full test suites incl. an
agent-loop round trip. ADR-0013; README skills section + matrix synced.

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

progress.md

@@ -1,5 +1,20 @@
 # progress
 
+## 2026-06-10 — Phase 6: skills
+
+**Landed:** `skill/` (ADR-0013): the agent.Skill contract satisfied by a
+buildable skill.New(name, WithInstructions/WithTools/WithToolbox);
+instruction-only skills legal; same-instance reuse across agents; additive
+ordered composition proven (prompt appending + toolset merge + loud
+duplicate policy). Example skills: `skill/clock` (time_now/time_convert,
+injectable clock) and `skill/calc` (calculate over a hand-rolled
+recursive-descent evaluator: + - * / % ^, parens, unary minus, scientific
+notation; division-by-zero and non-finite results rejected). Tests cover
+the evaluator table, tool execution through ExecuteTool, and a full
+agent-loop run answering from the calculate result.
+
+**Next:** Phase 7 — examples/, mort migration map, README finalization.
+
 ## 2026-06-10 — Phase 5: agent loop, Generate[T], schema derivation
 
 **Landed:** `agent/` (ADR-0012): New(model, system, opts) with toolboxes,