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/claude_test.go

@@ -72,6 +72,22 @@ func TestCheckDocumentationBdPrimeReference(t *testing.T) {
 			expectedStatus: "ok",
 			expectDetail:   true,
 		},
+		{
+			name: "claude.local.md references bd prime (local-only)",
+			fileContent: map[string]string{
+				"claude.local.md": "Run bd prime for context.",
+			},
+			expectedStatus: "ok",
+			expectDetail:   true,
+		},
+		{
+			name: ".claude/claude.local.md references bd prime (local-only)",
+			fileContent: map[string]string{
+				".claude/claude.local.md": "Use bd prime for workflow context.",
+			},
+			expectedStatus: "ok",
+			expectDetail:   true,
+		},
 		{
 			name: "multiple files reference bd prime",
 			fileContent: map[string]string{