feat(knowledge-base): define KB file format, entry rules, and categories (#8) (#51)

Formalize the Knowledge Base on-disk format and tighten the entry schema per
MASTER_SPEC §15.3-§15.10, §18.2.

- compiler/common/categories.py: single source of truth for the 19 canonical
  categories (lowercase identifiers of the §18.2 list) plus is_valid_category.
- knowledge_base_entry.schema.json (v1.1): category restricted to the 19; tags
  require at least one item; expand entries must be snake_case canonical ids;
  aliases uniqueItems.
- knowledge_base/README.md: by-domain file layout, the "each file is a JSON array
  of entries" format, and contributor field rules (id/alias global uniqueness,
  tag order, expansion targets, one-tag minimum).
- examples/knowledge_base.example.json: worked file demonstrating an alias and an
  expansion.

Cross-entry rules (global uniqueness, expand targets exist) are deferred to the
KB validation tooling (#9); this issue defines per-entry structure only.

Tests: 19-category constant and schema-enum consistency, entry validation
(category/tags/id/expand/unknown-field), and the worked example (array shape,
per-entry validity, alias+expansion present, expand targets resolve).

Co-authored-by: kunail0804 <kunail0804@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

examples/knowledge_base.example.json

@@ -0,0 +1,16 @@
+[
+  {
+    "id": "long_hair",
+    "aliases": ["lengthy hair"],
+    "tags": ["long hair"],
+    "category": "hair"
+  },
+  {
+    "id": "twin_braids",
+    "aliases": ["twintail braids", "double braids"],
+    "tags": ["twin braids"],
+    "category": "hair",
+    "expand": ["long_hair"],
+    "notes": "Composite hairstyle: resolving 'twin braids' also adds 'long hair' via expansion."
+  }
+]