1.0 quality polish — code review, tests, bug fixes, documentation (#32)

Comprehensive quality pass targeting 1.0 release:

- Code review and refactoring
- Fix open bugs (#14, #16, #23)
- Expand test coverage
- Lint clean
- README update with build instructions (#9)
- Documentation improvements

Branched from `next` (active dev branch).

Reviewed-on: #32
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/cli/export.go

@@ -0,0 +1,72 @@
+package cli
+
+import (
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+	"time"
+
+	"github.com/urfave/cli/v2"
+	"sneak.berlin/go/mfer/mfer"
+)
+
+// ExportEntry represents a single file entry in the exported JSON output.
+type ExportEntry struct {
+	Path   string   `json:"path"`
+	Size   int64    `json:"size"`
+	Hashes []string `json:"hashes"`
+	Mtime  *string  `json:"mtime,omitempty"`
+	Ctime  *string  `json:"ctime,omitempty"`
+}
+
+func (mfa *CLIApp) exportManifestOperation(ctx *cli.Context) error {
+	pathOrURL, err := mfa.resolveManifestArg(ctx)
+	if err != nil {
+		return fmt.Errorf("export: %w", err)
+	}
+
+	rc, err := mfa.openManifestReader(pathOrURL)
+	if err != nil {
+		return fmt.Errorf("export: %w", err)
+	}
+	defer func() { _ = rc.Close() }()
+
+	manifest, err := mfer.NewManifestFromReader(rc)
+	if err != nil {
+		return fmt.Errorf("export: failed to parse manifest: %w", err)
+	}
+
+	files := manifest.Files()
+	entries := make([]ExportEntry, 0, len(files))
+
+	for _, f := range files {
+		entry := ExportEntry{
+			Path:   f.Path,
+			Size:   f.Size,
+			Hashes: make([]string, 0, len(f.Hashes)),
+		}
+
+		for _, h := range f.Hashes {
+			entry.Hashes = append(entry.Hashes, hex.EncodeToString(h.MultiHash))
+		}
+
+		if f.Mtime != nil {
+			t := time.Unix(f.Mtime.Seconds, int64(f.Mtime.Nanos)).UTC().Format(time.RFC3339Nano)
+			entry.Mtime = &t
+		}
+		if f.Ctime != nil {
+			t := time.Unix(f.Ctime.Seconds, int64(f.Ctime.Nanos)).UTC().Format(time.RFC3339Nano)
+			entry.Ctime = &t
+		}
+
+		entries = append(entries, entry)
+	}
+
+	enc := json.NewEncoder(mfa.Stdout)
+	enc.SetIndent("", "  ")
+	if err := enc.Encode(entries); err != nil {
+		return fmt.Errorf("export: failed to encode JSON: %w", err)
+	}
+
+	return nil
+}