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>

test/fuzz.sh

@@ -0,0 +1,37 @@
+#!/bin/bash
+#
+# Run fuzz tests for cheat
+# Usage: ./scripts/fuzz.sh [duration]
+#
+# Note: Go's fuzzer will fail immediately if it finds a known failing input
+# in the corpus (testdata/fuzz/*). This is by design - it ensures you fix
+# known bugs before searching for new ones. To see failing inputs:
+#   ls internal/*/testdata/fuzz/*/
+#
+
+set -e
+
+DURATION="${1:-15s}"
+
+# Define fuzz tests: "TestName:Package:Description"
+TESTS=(
+    "FuzzParse:./internal/sheet:YAML frontmatter parsing"
+    "FuzzValidate:./internal/sheet:sheet name validation (path traversal protection)"
+    "FuzzSearchRegex:./internal/sheet:regex search operations"
+    "FuzzTagged:./internal/sheet:tag matching with malicious input"
+    "FuzzFilter:./internal/sheets:tag filtering operations"
+    "FuzzTags:./internal/sheets:tag aggregation and sorting"
+    "FuzzFindLocalCheatpath:./internal/config:recursive .cheat directory discovery"
+)
+
+echo "Running fuzz tests ($DURATION each)..."
+echo
+
+for i in "${!TESTS[@]}"; do
+    IFS=':' read -r test_name package description <<< "${TESTS[$i]}"
+    echo "$((i+1)). Testing $description..."
+    go test -fuzz="^${test_name}$" -fuzztime="$DURATION" "$package"
+    echo
+done
+
+echo "All fuzz tests passed!"