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>

README.md

@@ -237,9 +237,32 @@ the model can react to; unknown tools likewise; duplicate tool names across
 toolboxes fail loudly. On `agent.ErrMaxSteps` (and on model errors) the
 partial result with the full transcript is still returned.
 
-## Skills *(pending — Phase 6)*
+## Skills
 
-Skills = reusable instruction+tool bundles attachable to any agent.
+Skills are reusable instruction+tool bundles attachable to **any** agent,
+at construction or on demand. Instructions extend the system prompt;
+tools extend the toolset — additively, in attachment order.
+
+```go
+import (
+    "gitea.stevedudenhoeffer.com/steve/majordomo/skill"
+    "gitea.stevedudenhoeffer.com/steve/majordomo/skill/calc"
+    "gitea.stevedudenhoeffer.com/steve/majordomo/skill/clock"
+)
+
+research := skill.New("research",
+    skill.WithInstructions("Cite a source for every claim."),
+    skill.WithTools(searchTool, fetchTool),
+)
+
+a := agent.New(m, "You are helpful.", agent.WithSkill(research))
+a.AddSkill(clock.New()) // ready-made: time awareness
+a.AddSkill(calc.New())  // ready-made: exact arithmetic
+```
+
+Anything implementing the three-method `agent.Skill` interface (Name /
+Instructions / Tools) is a skill — `skill.New` is just the convenient way
+to build one.
 
 ## Feature/provider support matrix
 
@@ -263,7 +286,7 @@ Notes: Ollama has no native tool_choice — `"none"` drops the tools;
 Cross-cutting: Parse grammar ✅ · aliases/tiers ✅ · failover chains ✅ ·
 health tracking/backoff ✅ · LLM_* env DSNs ✅ · media pipeline ✅
 (per-target normalization in chains) · agent loop ✅ · `Generate[T]` +
-schema derivation ✅ · skills pending.
+schema derivation ✅ · skills ✅ (with clock + calc examples).
 
 ## Development