feat: subtype/generalization via subtypeOf (#8)

[jbeda]

Fourth feature from the schema-expressiveness pass. Implements ADR-0004; additive to schema v1 (goldens unchanged — examples have no subtypeOf).

Closes #8.

What changed

• subtypeOf on an entity (PascalCase, references a defined entity) — an is-a / generalization link the child declares. Schema + Entity.SubtypeOf.
• Lint (runSubtypes): errors on a subtypeOf naming an undefined entity, and on a self- or transitively-cyclic chain (cycle-safe walk, no infinite loop). Completeness now treats a parent's invariants as covering its subtypes — a subtype that adds no rule of its own is no longer flagged "has no invariants" when an ancestor has them (transitive, cycle-safe).
• Render (Markdown): each child names its supertype (Subtype of \Parent``); each parent lists its subtypes (sorted, deterministic). Per ADR-0002/0004 the Mermaid ER draws no is-a edge — erDiagram has no generalization notation, so the hierarchy lives in the Markdown.
• Docs: schema-reference documents subtypeOf and the ER limitation.

Process (agent-workflow.md)

Correctness-critical (schema, struct-sync, lint semantics, completeness change), so a full adversarial round (Opus) ran against hostile fixtures (self/mutual/3-cycles, cycle-not-at-root, transitive inheritance chains, undefined parents, deep acyclic chains).

Result: clean on correctness. One cosmetic fix applied — a node pointing into a cycle was told it "is a subtype of itself"; reworded to be accurate for both cases. One note accepted as-is (an undefined parent yields both the undefined error and a "no invariants" warning — each independently true). Record: .scratch/reviews/subtype.md.

Verification

task check passes end to end (vet, staticcheck, golangci-lint 0 issues, -race tests, lint-models, render-check, validate-plugin). TestSchemaStructSync/TestURLConsistency hold; render --check clean on both fixtures (no drift).

Tests: TestADR_0004_UndefinedSupertype, TestADR_0004_SubtypeInheritsInvariants, TestSubtypeCycleDetected, TestRenderEntity_SubtypeHierarchy.

Also sneaks in a .gitignore entry for /.claude/worktrees/ (subagent worktrees should never be committed).

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