feat(messages): include files field in message JSON output (#120)

## Summary

- Add `File` struct and `Files` field to `Message` so file attachment
metadata from the Slack API is preserved in JSON output
- Covers `messages thread`, `messages history`, and any other command
returning `Message` objects

## Why

The `Message` struct had no `Files` field, so file attachment data
returned by the Slack API (`conversations.replies`,
`conversations.history`) was silently dropped during JSON unmarshalling.
A message like "see the screenshot below" appeared identical to a plain
text message in `--output json`, losing important context for automated
consumers.

## Test plan

- [x] New test `TestRunThread_JSONIncludesFiles` verifies files
round-trip through JSON output
- [x] Existing tests pass (reactions, history, thread)
- [x] Linter clean (`0 issues`)

Closes #119

Author: piekstra

internal/client/client.go

@@ -230,6 +230,19 @@ type Reaction struct {
 	Users []string `json:"users"`
 }
 
+// File represents a file attachment on a Slack message
+type File struct {
+	ID                 string `json:"id"`
+	Name               string `json:"name"`
+	Title              string `json:"title,omitempty"`
+	Mimetype           string `json:"mimetype,omitempty"`
+	Filetype           string `json:"filetype,omitempty"`
+	Size               int64  `json:"size,omitempty"`
+	URLPrivate         string `json:"url_private,omitempty"`
+	URLPrivateDownload string `json:"url_private_download,omitempty"`
+	Permalink          string `json:"permalink,omitempty"`
+}
+
 // Message represents a Slack message
 type Message struct {
 	Type       string     `json:"type"`
@@ -239,6 +252,7 @@ type Message struct {
 	ThreadTS   string     `json:"thread_ts,omitempty"`
 	ReplyCount int        `json:"reply_count,omitempty"`
 	Reactions  []Reaction `json:"reactions,omitempty"`
+	Files      []File     `json:"files,omitempty"`
 }
 
 // Team represents workspace info

internal/cmd/messages/messages_test.go

@@ -543,6 +543,79 @@ func TestRunThread_JSONIncludesReactions(t *testing.T) {
 	assert.Empty(t, messages[1].Reactions)
 }
 
+func TestRunThread_JSONIncludesFiles(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.URL.Path {
+		case "/conversations.replies":
+			_ = json.NewEncoder(w).Encode(map[string]interface{}{
+				"ok": true,
+				"messages": []map[string]interface{}{
+					{
+						"ts":   "1234567890.123456",
+						"user": "U001",
+						"text": "See the screenshot below",
+						"files": []map[string]interface{}{
+							{
+								"id":                   "F0123ABC",
+								"name":                 "screenshot.png",
+								"title":                "Screenshot",
+								"mimetype":             "image/png",
+								"filetype":             "png",
+								"size":                 54321,
+								"url_private":          "https://files.slack.com/files-pri/T123-F0123ABC/screenshot.png",
+								"url_private_download": "https://files.slack.com/files-pri/T123-F0123ABC/download/screenshot.png",
+								"permalink":            "https://example.slack.com/files/U001/F0123ABC/screenshot.png",
+							},
+						},
+					},
+					{
+						"ts":   "1234567890.123457",
+						"user": "U002",
+						"text": "Plain reply without files",
+					},
+				},
+			})
+		case "/users.info":
+			mockUserInfoHandler(w, r)
+		}
+	}))
+	defer server.Close()
+
+	c := client.NewWithConfig(server.URL, "test-token", nil)
+	opts := &threadOptions{limit: 100}
+
+	// Capture JSON output
+	output.OutputFormat = output.FormatJSON
+	defer func() { output.OutputFormat = output.FormatText }()
+
+	var buf strings.Builder
+	output.Writer = &buf
+	defer func() { output.Writer = os.Stdout }()
+
+	err := runThread("C123", "1234567890.123456", opts, c)
+	require.NoError(t, err)
+
+	// Parse the JSON output
+	var messages []client.Message
+	err = json.Unmarshal([]byte(buf.String()), &messages)
+	require.NoError(t, err)
+
+	require.Len(t, messages, 2)
+
+	// First message should have files
+	require.Len(t, messages[0].Files, 1)
+	assert.Equal(t, "F0123ABC", messages[0].Files[0].ID)
+	assert.Equal(t, "screenshot.png", messages[0].Files[0].Name)
+	assert.Equal(t, "image/png", messages[0].Files[0].Mimetype)
+	assert.Equal(t, "png", messages[0].Files[0].Filetype)
+	assert.Equal(t, int64(54321), messages[0].Files[0].Size)
+	assert.Equal(t, "https://files.slack.com/files-pri/T123-F0123ABC/screenshot.png", messages[0].Files[0].URLPrivate)
+	assert.Equal(t, "https://example.slack.com/files/U001/F0123ABC/screenshot.png", messages[0].Files[0].Permalink)
+
+	// Second message should have no files
+	assert.Empty(t, messages[1].Files)
+}
+
 func TestRunHistory_JSONIncludesReactions(t *testing.T) {
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		switch r.URL.Path {