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>

docs/adr/0013-skill-model.md

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+# ADR-0013: Skill model — additive instruction+tool bundles
+
+**Status:** Accepted — 2026-06-10
+
+## Context
+
+mort's "skills" are reusable capabilities (a prompt fragment plus tools)
+attached to different agents at runtime. majordomo needs the same shape
+without mort specifics: attachable to any agent, on demand, composing
+additively and predictably.
+
+## Decision
+
+- The **contract** is the three-method `agent.Skill` interface (Name /
+  Instructions / Tools). It lives in package agent — the consumer of the
+  contract — so skill→agent and agent→skill never both happen (no import
+  cycle, and third parties can satisfy it without importing package skill).
+- Package **skill** is the standard constructor: `skill.New(name,
+  WithInstructions(...), WithTools(...)/WithToolbox(...))`. Instruction-only
+  skills (nil toolbox) are legal — agent treats Tools()==nil as "no tools".
+- **Composition is additive and ordered:** instructions append to the
+  agent's system prompt in attachment order, separated by blank lines;
+  skill tools join the merged toolset with the same loud duplicate-name
+  policy as toolboxes (ADR-0012). No hooks, no resources, no lifecycle —
+  none of mort's skills need them, and out-of-scope discipline says don't
+  build for hypotheticals (they'd be an additive interface upgrade later).
+- Two ready-made example skills ship as subpackages and double as the
+  pattern reference: `skill/clock` (time awareness; injectable clock) and
+  `skill/calc` (exact arithmetic via a hand-rolled recursive-descent
+  evaluator — no eval, no dependencies).
+
+## Consequences
+
+- A skill instance is reusable across agents simultaneously (it is
+  read-only after construction).
+- mort's skill registry maps onto plain values: build `*skill.Skill`s,
+  attach per agent at invocation time.