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/cheatpath/cheatpath.go

@@ -2,27 +2,10 @@
 // management.
 package cheatpath
 
-import "fmt"
-
-// Cheatpath encapsulates cheatsheet path information
-type Cheatpath struct {
+// Path encapsulates cheatsheet path information
+type Path struct {
 	Name     string   `yaml:"name"`
 	Path     string   `yaml:"path"`
 	ReadOnly bool     `yaml:"readonly"`
 	Tags     []string `yaml:"tags"`
 }
-
-// Validate ensures that the Cheatpath is valid
-func (c Cheatpath) Validate() error {
-	// Check that name is not empty
-	if c.Name == "" {
-		return fmt.Errorf("cheatpath name cannot be empty")
-	}
-
-	// Check that path is not empty
-	if c.Path == "" {
-		return fmt.Errorf("cheatpath path cannot be empty")
-	}
-
-	return nil
-}