fix(best-practices): restore canonical content, dedupe, sync script, spec exemplar

- best-practices/v1: replace 9 stale symlinks (into planning/v1) with real files
  synced byte-identical from the canonical best-practices project; add INDEX.md,
  scripting.md, mechanical-test-generation.md (canonical had drifted heavily,
  e.g. api-design.md 463->807 lines)
- planning/v1: delete duplicated best-practices/ copy (requires: inheritance
  confirmed via spec/harness.md HC-1/HC-7)
- scripts/sync-best-practices.sh: idempotent re-sync from canonical checkout
- code-methodology/v1: INDEX.md + scripting.md references now resolve; point
  test-writing tasks at mechanical-test-generation.md
- spec-writing/v1: worked spec exemplar (module layout table, Why: lines, exact
  error messages, parametrize pattern table) + CLAUDE.md pointer + mount entry

harnesses/contexts/spec-writing/v1/harness.yaml

@@ -9,3 +9,5 @@ provides: [spec-writer]
 context_files:
   - source: ./CLAUDE.md
     target: /opt/harness/context/spec-writing/CLAUDE.md
+  - source: ./spec-exemplar.md
+    target: /opt/harness/context/spec-writing/spec-exemplar.md