chore: housekeeping and refactoring (bump to 4.7.1)

- Remove unused parameters, dead files, and inaccurate doc.go files
- Extract shared helpers, eliminate duplication
- Rename cheatpath.Cheatpath to cheatpath.Path
- Optimize filesystem walks (WalkDir, skip .git)
- Move sheet name validation to sheet.Validate
- Move integration tests to test/integration/
- Consolidate internal/mock into mocks/
- Move fuzz.sh to test/
- Inline loadSheets helper into command callers
- Extract config.New into its own file
- Fix stale references in HACKING.md and CLAUDE.md
- Restore plan9 build target
- Remove redundant and low-value tests
- Clean up project documentation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/sheet/parse_extended_test.go

@@ -27,22 +27,3 @@ func TestParseWindowsLineEndings(t *testing.T) {
 		t.Errorf("failed to parse syntax: want: %s, got: %s", want, fm.Syntax)
 	}
 }
-
-// TestParseInvalidYAML tests parsing with invalid YAML in frontmatter
-func TestParseInvalidYAML(t *testing.T) {
-	// stub our cheatsheet content with invalid YAML
-	markdown := `---
-syntax: go
-tags: [ test
-  unclosed bracket
----
-To foo the bar: baz`
-
-	// parse the frontmatter
-	_, _, err := parse(markdown)
-
-	// assert that an error was returned for invalid YAML
-	if err == nil {
-		t.Error("expected error for invalid YAML, got nil")
-	}
-}