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/writeable_test.go

@@ -9,10 +9,10 @@ import (
 func TestWriteableOK(t *testing.T) {
 
 	// initialize some cheatpaths
-	cheatpaths := []Cheatpath{
-		Cheatpath{Path: "/foo", ReadOnly: true},
-		Cheatpath{Path: "/bar", ReadOnly: false},
-		Cheatpath{Path: "/baz", ReadOnly: true},
+	cheatpaths := []Path{
+		Path{Path: "/foo", ReadOnly: true},
+		Path{Path: "/bar", ReadOnly: false},
+		Path{Path: "/baz", ReadOnly: true},
 	}
 
 	// get the writeable cheatpath
@@ -34,10 +34,10 @@ func TestWriteableOK(t *testing.T) {
 func TestWriteableNotOK(t *testing.T) {
 
 	// initialize some cheatpaths
-	cheatpaths := []Cheatpath{
-		Cheatpath{Path: "/foo", ReadOnly: true},
-		Cheatpath{Path: "/bar", ReadOnly: true},
-		Cheatpath{Path: "/baz", ReadOnly: true},
+	cheatpaths := []Path{
+		Path{Path: "/foo", ReadOnly: true},
+		Path{Path: "/bar", ReadOnly: true},
+		Path{Path: "/baz", ReadOnly: true},
 	}
 
 	// get the writeable cheatpath