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>

skill/skill.go

@@ -0,0 +1,66 @@
+// Package skill provides reusable capability bundles for agents: a named
+// set of instructions (appended to the agent's system prompt) plus optional
+// tools (joined into the agent's toolset). Skills attach to ANY agent, at
+// construction (agent.WithSkill) or on demand (Agent.AddSkill), and compose
+// additively in attachment order.
+//
+// The contract a skill satisfies is the agent.Skill interface; this package
+// is the standard way to build one without writing a type:
+//
+//	research := skill.New("research",
+//	    skill.WithInstructions("Cite sources for every claim."),
+//	    skill.WithTools(searchTool, fetchTool),
+//	)
+//	a.AddSkill(research)
+//
+// Two ready-made example skills ship as subpackages: clock (time awareness)
+// and calc (exact arithmetic).
+package skill
+
+import "gitea.stevedudenhoeffer.com/steve/majordomo/llm"
+
+// Skill is a buildable instruction+tool bundle. The zero value is unusable;
+// construct with New.
+type Skill struct {
+	name         string
+	instructions string
+	toolbox      *llm.Toolbox
+}
+
+// Option configures a Skill under construction.
+type Option func(*Skill)
+
+// WithInstructions sets the text appended to the agent's system prompt.
+func WithInstructions(s string) Option {
+	return func(sk *Skill) { sk.instructions = s }
+}
+
+// WithToolbox attaches the skill's tools as an existing toolbox.
+func WithToolbox(b *llm.Toolbox) Option {
+	return func(sk *Skill) { sk.toolbox = b }
+}
+
+// WithTools attaches loose tools (wrapped in a toolbox named after the
+// skill).
+func WithTools(tools ...llm.Tool) Option {
+	return func(sk *Skill) { sk.toolbox = llm.NewToolbox(sk.name, tools...) }
+}
+
+// New builds a skill.
+func New(name string, opts ...Option) *Skill {
+	sk := &Skill{name: name}
+	for _, opt := range opts {
+		opt(sk)
+	}
+	return sk
+}
+
+// Name identifies the skill (used in duplicate-tool diagnostics).
+func (s *Skill) Name() string { return s.name }
+
+// Instructions returns the system-prompt extension; may be empty.
+func (s *Skill) Instructions() string { return s.instructions }
+
+// Tools returns the skill's toolbox; may be nil for instruction-only
+// skills.
+func (s *Skill) Tools() *llm.Toolbox { return s.toolbox }