Fixes #135

Root cause:
- sqlite_extension.go and sqlite_vec_loader.go use CGO-only APIs
  (sqlite3.SQLiteConn.LoadExtension)
- Docker builds use CGO_ENABLED=0 for static compilation
- These files were being compiled in non-CGO environment, causing:
  "LoadExtension undefined (type *sqlite3.SQLiteConn has no field or method)"

Solution:
Use build tags to separate CGO and non-CGO implementations:

1. sqlite_extension.go: !sqlite_vec && cgo
   - CGO environment: Load extension from dynamic library (.so/.dylib)
   - Non-CGO: Skip this file

2. sqlite_vec_loader.go: !sqlite_vec && cgo
   - CGO environment: Try multiple extension paths
   - Non-CGO: Skip this file

3. sqlite_extension_stub.go: !sqlite_vec && !cgo (NEW)
   - Non-CGO environment: Return error (use Go fallback)
   - CGO: Skip this file

Build matrix:
+------------+------------+----------------------+
| sqlite_vec | CGO       | Implementation        |
+------------+------------+----------------------+
| false      | true      | Dynamic loader       |
| false      | false     | Stub (Go fallback)   |
| true       | N/A       | Static linking       |
+------------+------------+----------------------+

Verification:
- ✅ CGO_ENABLED=0 build successful (Docker static build)
- ✅ CGO_ENABLED=1 build successful (standard build)
- ✅ sqlite-vec extension loads correctly (18 functions)
- ✅ Local testing passed
- ✅ No regression in existing functionality

Impact:
- Docker images can now build successfully
- Non-CGO builds use Go fallback for vector search
- CGO builds maintain full sqlite-vec support

Refs #135
Refs #131

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>