fix: improve package search relevance with exact match priority

Package search now prioritizes results in this order:
1. Exact pname match
2. Exact attr_path match
3. pname starts with query
4. attr_path starts with query
5. FTS ranking (bm25 for SQLite, ts_rank for PostgreSQL)

This ensures searching for "git" returns the "git" package first,
rather than packages that merely mention "git" in their description.

Also update CLAUDE.md to clarify using `nix run` instead of
`go build -o` for testing binaries.

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

CLAUDE.md

@@ -223,7 +223,9 @@ hm-options --version                   # Show version
 ### Development Workflow
 - **Always run `go fmt ./...` before committing Go code**
 - **Run Go commands using `nix develop -c`** (e.g., `nix develop -c go test ./...`)
-- **Use `nix run` to run binaries** (e.g., `nix run .#nixpkgs-search -- options serve`)
+- **Use `nix run` to run/test binaries** (e.g., `nix run .#nixpkgs-search -- options serve`)
+  - Do NOT use `go build -o /tmp/...` to test binaries - always use `nix run`
+  - Remember: modified files must be tracked by git for `nix run` to see them
 - File paths in responses should use format `path/to/file.go:123`
 
 ### Linting