feat: support claude.local.md for local-only documentation

Add support for claude.local.md and .claude/claude.local.md as local-only
alternatives to CLAUDE.md. These files are intended to be gitignored for
personal customizations that should not be committed to the repo.

Changes:
- Update CheckAgentDocumentation to detect claude.local.md files
- Update CheckLegacyBeadsSlashCommands to check claude.local.md files
- Update CheckDocumentationBdPrimeReference to check claude.local.md files
- Add fix message mentioning claude.local.md as an option
- Add test cases for claude.local.md support

Closes #488

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/doctor/legacy_test.go

@@ -39,6 +39,18 @@ func TestCheckAgentDocumentation(t *testing.T) {
 			expectedStatus: "ok",
 			expectFix:      false,
 		},
+		{
+			name:           "claude.local.md exists (local-only)",
+			files:          []string{"claude.local.md"},
+			expectedStatus: "ok",
+			expectFix:      false,
+		},
+		{
+			name:           ".claude/claude.local.md exists (local-only)",
+			files:          []string{".claude/claude.local.md"},
+			expectedStatus: "ok",
+			expectFix:      false,
+		},
 		{
 			name:           "multiple docs",
 			files:          []string{"AGENTS.md", "CLAUDE.md"},
@@ -127,6 +139,22 @@ func TestCheckLegacyBeadsSlashCommands(t *testing.T) {
 			expectedStatus: "warning",
 			expectWarning:  true,
 		},
+		{
+			name: "legacy slash command in claude.local.md",
+			fileContent: map[string]string{
+				"claude.local.md": "Use /beads:show to see an issue.",
+			},
+			expectedStatus: "warning",
+			expectWarning:  true,
+		},
+		{
+			name: "legacy slash command in .claude/claude.local.md",
+			fileContent: map[string]string{
+				".claude/claude.local.md": "Use /beads:ready to see ready issues.",
+			},
+			expectedStatus: "warning",
+			expectWarning:  true,
+		},
 		{
 			name: "multiple files with legacy commands",
 			fileContent: map[string]string{