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/config/new_test.go

@@ -38,7 +38,7 @@ cheatpaths:
 	}
 
 	// Load the config
-	conf, err := New(map[string]interface{}{}, configPath, false)
+	conf, err := New(configPath, false)
 	if err != nil {
 		t.Fatalf("failed to load config: %v", err)
 	}
@@ -88,7 +88,7 @@ cheatpaths:
 	}
 
 	// Load the config
-	conf, err := New(map[string]interface{}{}, configPath, false)
+	conf, err := New(configPath, false)
 	if err != nil {
 		// It's OK if this fails due to no editor being found
 		// The important thing is it doesn't panic
@@ -123,7 +123,7 @@ cheatpaths:
 	}
 
 	// Load the config
-	conf, err := New(map[string]interface{}{}, configPath, false)
+	conf, err := New(configPath, false)
 	if err != nil {
 		t.Fatalf("failed to load config: %v", err)
 	}