test: Add security and error handling tests for lint warnings

Added comprehensive tests to address gosec and errcheck linter warnings:

1. bd-yxy (P0): Command injection prevention tests for git rm in merge command
   - Added merge_security_test.go with tests for shell metacharacters
   - Verified exec.Command safely passes arguments (no shell interpretation)
   - Added #nosec G204 comment explaining why code is safe

2. bd-nbc (P1): Security tests for file path validation in clean command
   - Added clean_security_test.go with path traversal tests
   - Verified filepath.Join safely constructs paths within .beads directory
   - Added #nosec G304 comment documenting safety guarantees

3. bd-lln (P2): Tests for performFlush error handling in FlushManager
   - Added tests documenting that performFlush intentionally returns nil
   - Errors are handled internally by flushToJSONLWithState
   - Tests verify graceful degradation when store is inactive

4. bd-gra (P2): Error handling test for cmd.Help() in search command
   - Added search_test.go documenting Help() error handling
   - Help() errors intentionally ignored (already in error path, will exit anyway)
   - Added #nosec G104 comment explaining rationale

All new tests pass. The linter warnings are false positives or intentional
design decisions, now documented with tests and #nosec comments.

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

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

cmd/bd/search.go

@@ -36,6 +36,9 @@ Examples:
 		// If no query provided, show help
 		if query == "" {
 			fmt.Fprintf(os.Stderr, "Error: search query is required\n")
+			// #nosec G104 -- cmd.Help() error intentionally ignored. We're already in an
+			// error path (missing query) and will exit(1) regardless. Help() errors are
+			// rare (I/O failures) and don't affect the outcome. See TestSearchCommand_HelpErrorHandling
 			cmd.Help()
 			os.Exit(1)
 		}