feat: walk up directory tree to find .cheat directory (#602)

Previously cheat only checked the current working directory for a .cheat
subdirectory. Now it walks upward through ancestor directories, stopping
at the first .cheat directory found. This mirrors how git discovers .git
directories, so users can place .cheat at their project root and have it
work from any subdirectory.

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

internal/config/config_fuzz_test.go

@@ -0,0 +1,122 @@
+package config
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// FuzzFindLocalCheatpath exercises findLocalCheatpath with randomised
+// directory depths and .cheat placements.  For each fuzz input it builds a
+// temporary directory hierarchy, places a single .cheat directory at a
+// computed level, and asserts that the function always returns it.
+func FuzzFindLocalCheatpath(f *testing.F) {
+	// Seed corpus: (totalDepth, cheatPlacement)
+	f.Add(uint8(1), uint8(0))  // depth 1, .cheat at root
+	f.Add(uint8(3), uint8(0))  // depth 3, .cheat at root
+	f.Add(uint8(5), uint8(3))  // depth 5, .cheat at level 3
+	f.Add(uint8(1), uint8(1))  // depth 1, .cheat at same level as search dir
+	f.Add(uint8(10), uint8(5)) // deep hierarchy
+
+	f.Fuzz(func(t *testing.T, totalDepth uint8, cheatPlacement uint8) {
+		// Clamp to reasonable values to keep I/O bounded
+		depth := int(totalDepth%15) + 1              // 1..15
+		cheatAt := int(cheatPlacement) % (depth + 1) // 0..depth (0 = tempDir itself)
+
+		tempDir := t.TempDir()
+
+		// Build chain: tempDir/d0/d1/…/d{depth-1}
+		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 dirs[cheatAt]
+		cheatDir := filepath.Join(dirs[cheatAt], ".cheat")
+		if err := os.Mkdir(cheatDir, 0755); err != nil {
+			t.Fatalf("mkdir .cheat: %v", err)
+		}
+
+		// Search from the deepest directory
+		result := findLocalCheatpath(current)
+
+		// Invariant 1: must find the .cheat we placed
+		if result != cheatDir {
+			t.Errorf("depth=%d cheatAt=%d: expected %s, got %s",
+				depth, cheatAt, cheatDir, result)
+		}
+
+		// Invariant 2: result must end with /.cheat
+		if !strings.HasSuffix(result, string(filepath.Separator)+".cheat") {
+			t.Errorf("result %q does not end with /.cheat", result)
+		}
+
+		// Invariant 3: result must be under tempDir
+		if !strings.HasPrefix(result, tempDir) {
+			t.Errorf("result %q is not under tempDir %s", result, tempDir)
+		}
+	})
+}
+
+// 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)
+		}
+	})
+}