refactor(zfc): derive state from files instead of in-memory cache

Apply ZFC (Zero Forge Cache) principle across git error handling and
feed curation. Agents now observe raw git output and make their own
decisions rather than relying on pre-interpreted error types.

- Add GitError type with raw stdout/stderr for observation
- Add SwarmGitError following the same pattern
- Remove in-memory deduplication maps from Curator
- Curator now reads state from feed/events files

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

internal/refinery/engineer.go

@@ -5,7 +5,6 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -312,7 +311,8 @@ 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 {
-		if errors.Is(err, git.ErrMergeConflict) {
+		// ZFC: Check for conflict via GitError method instead of sentinel error
+		if gitErr, ok := err.(*git.GitError); ok && gitErr.HasConflict() {
 			_ = e.git.AbortMerge()
 			return ProcessResult{
 				Success:  false,