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/merge.go

@@ -118,6 +118,9 @@ func cleanupMergeArtifacts(outputPath string, debug bool) {
 			fullPath := filepath.Join(beadsDir, entry.Name())
 
 			// Try to git rm if tracked
+			// #nosec G204 -- fullPath is safely constructed via filepath.Join from entry.Name()
+			// from os.ReadDir. exec.Command does NOT use shell interpretation - arguments
+			// are passed directly to git binary. See TestCleanupMergeArtifacts_CommandInjectionPrevention
 			gitRmCmd := exec.Command("git", "rm", "-f", "--quiet", fullPath)
 			gitRmCmd.Dir = filepath.Dir(beadsDir)
 			_ = gitRmCmd.Run() // Ignore errors, file may not be tracked