fix(zfc): remove strings.Contains conflict detection from Go code

hq-hcil1: Remove deprecated HasConflict/HasAuthFailure/IsNotARepo/HasRebaseConflict
methods that violated ZFC by having Go code decide error types based on stderr parsing.

Changes:
- Remove deprecated helper methods from GitError and SwarmGitError
- Export GetConflictingFiles() which uses git porcelain output (diff --diff-filter=U)
- Update CheckConflicts(), engineer.go, and integration.go to use GetConflictingFiles()
- Update tests to verify raw stderr is available for agent observation

ZFC principle: Go code transports raw output to agents; agents observe and decide.

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

internal/refinery/engineer.go

@@ -311,8 +311,10 @@ func (e *Engineer) doMerge(ctx context.Context, branch, target, sourceIssue stri
 	}
 	_, _ = fmt.Fprintf(e.output, "[Engineer] Merging with message: %s\n", mergeMsg)
 	if err := e.git.MergeNoFF(branch, mergeMsg); err != nil {
-		// ZFC: Check for conflict via GitError method instead of sentinel error
-		if gitErr, ok := err.(*git.GitError); ok && gitErr.HasConflict() {
+		// ZFC: Use git's porcelain output to detect conflicts instead of parsing stderr.
+		// GetConflictingFiles() uses `git diff --diff-filter=U` which is proper.
+		conflicts, conflictErr := e.git.GetConflictingFiles()
+		if conflictErr == nil && len(conflicts) > 0 {
 			_ = e.git.AbortMerge()
 			return ProcessResult{
 				Success:  false,