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/config_fuzz_test.go

@@ -65,58 +65,3 @@ func FuzzFindLocalCheatpath(f *testing.F) {
 		}
 	})
 }
-
-// FuzzFindLocalCheatpathNearestWins verifies that when two .cheat directories
-// exist at different levels of the ancestor chain, the nearest one is returned.
-func FuzzFindLocalCheatpathNearestWins(f *testing.F) {
-	f.Add(uint8(5), uint8(1), uint8(3))
-	f.Add(uint8(8), uint8(0), uint8(7))
-	f.Add(uint8(3), uint8(0), uint8(2))
-	f.Add(uint8(10), uint8(2), uint8(8))
-
-	f.Fuzz(func(t *testing.T, totalDepth, shallowRaw, deepRaw uint8) {
-		depth := int(totalDepth%12) + 2 // 2..13 (need room for two placements)
-		s := int(shallowRaw) % depth
-		d := int(deepRaw) % depth
-
-		// Need two distinct levels
-		if s == d {
-			d = (d + 1) % depth
-		}
-		// Ensure s < d (shallow is higher in tree, deep is closer to search dir)
-		if s > d {
-			s, d = d, s
-		}
-
-		tempDir := t.TempDir()
-
-		// Build chain
-		dirs := make([]string, 0, depth+1)
-		dirs = append(dirs, tempDir)
-		current := tempDir
-		for i := 0; i < depth; i++ {
-			current = filepath.Join(current, fmt.Sprintf("d%d", i))
-			if err := os.Mkdir(current, 0755); err != nil {
-				t.Fatalf("mkdir: %v", err)
-			}
-			dirs = append(dirs, current)
-		}
-
-		// Place .cheat at both levels
-		shallowCheat := filepath.Join(dirs[s], ".cheat")
-		deepCheat := filepath.Join(dirs[d], ".cheat")
-		if err := os.Mkdir(shallowCheat, 0755); err != nil {
-			t.Fatalf("mkdir shallow .cheat: %v", err)
-		}
-		if err := os.Mkdir(deepCheat, 0755); err != nil {
-			t.Fatalf("mkdir deep .cheat: %v", err)
-		}
-
-		// Search from the deepest directory — should find the deeper (nearer) .cheat
-		result := findLocalCheatpath(current)
-		if result != deepCheat {
-			t.Errorf("depth=%d shallow=%d deep=%d: expected nearest %s, got %s",
-				depth, s, d, deepCheat, result)
-		}
-	})
-}