p0-1: wire-schema stubs, doc/spec stubs, V001 migration, fixtures

- docs/wire-schema/v1/ ships 7 schema stubs (citation, search_hit,
  answer, ingest_report, doc_summary, chunk_inspection, doctor) that
  pin schema_version + required fields per design §2. Full property
  validation lands in later phases.
- docs/spec/ ships 7 markdown stubs each linking to the canonical
  frozen design (domain-model, ids, canonical-document, chunk-policy,
  citation-policy, module-boundaries, ai-generation-guidelines).
- migrations/V001__init.sql contains only schema_meta + migrations
  tables per design §5.1; data tables ship in P1-6/P2-1/P3-3.
- fixtures/ has the 11 subdirectories every downstream task references
  (markdown, source-fs, search/{lexical,hybrid}, embed, vector, rag,
  eval, image, pdf, audio). Empty subdirs use .gitkeep so they track.
  fixtures/markdown/ ships the 3 phase-0 fixtures: simple-note.md,
  nested-headings.md, code-and-table.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/spec/chunk-policy.md

@@ -0,0 +1,8 @@
+# Chunk policy
+
+`ChunkPolicy` carries `target_tokens`, `overlap_tokens`,
+`respect_markdown_headings`, and `chunker_version`. Chunkers expose a
+`policy_hash` so chunk IDs include the policy.
+
+Canonical source:
+[docs/superpowers/specs/2026-04-27-kb-final-form-design.md](../superpowers/specs/2026-04-27-kb-final-form-design.md), §3.5 + §7.1 + §7.2.