feat: extract email metadata from file-type messages

[derodero24]

Summary

• Add FilesToText() to extract email metadata (From, CC, Subject) from files[] when msg.Text is empty
• Email messages forwarded to Slack channels store content in files[] with filetype: "email", resulting in completely empty text output — this PR fills that gap

Ref #191 (partial fix — metadata only; email body requires upstream dependency update)

Before / After

Slack API response (forwarded email)

{
  "text": "",
  "files": [{
    "filetype": "email",
    "subject": "Meeting Tomorrow",
    "from": [{"name": "John Doe", "address": "[email protected]"}],
    "cc": [{"address": "[email protected]"}]
  }]
}

conversations_history CSV output

Before:

MsgID,UserID,UserName,RealName,Channel,ThreadTs,Text,Time,BotName,Cursor
1770523338.574369,USLACKBOT,Email,Email,C001,,,2026-02-08T04:02:18Z,Email,

After:

MsgID,UserID,UserName,RealName,Channel,ThreadTs,Text,Time,BotName,Cursor
1770523338.574369,USLACKBOT,Email,Email,C001,,Email, From: John Doe - john at example.com, CC: team at example.com, Subject: Meeting Tomorrow,2026-02-08T04:02:18Z,Email,

Design decisions

• Format separators chosen to survive filterSpecialChars (, instead of ;, at instead of @, / between CC recipients)
• To / BCC omitted — To contains Slack internal routing addresses (e.g. Email <>); BCC is not included in the Slack API response
• Email body not included — the slack-go/slack File struct does not yet map the plain_text field; this will be possible after feat: add PlainText and PreviewPlainText fields to File struct slack-go/slack#1522 is released (see feat: extract email metadata from file-type messages #191 for details)
• conversations_search not covered — Slack's SearchMessage type has no Files field; only conversations_history and conversations_replies are supported

Conflict with #190

This PR conflicts with #190 (Block Kit text extraction) at conversations.go L702, since both modify the same fallback logic. The resolution is to chain the fallbacks:

msgText := msg.Text
if msgText == "" {
    msgText = text.BlocksToText(msg.Blocks)   // #190
}
if msgText == "" {
    msgText = text.FilesToText(msg.Files)      // this PR
}
msgText += text.AttachmentsTo2CSV(msgText, msg.Attachments)

Whichever PR merges first, the other can be rebased with this 3-line addition.

Test plan

• 7 unit tests for FilesToText (filtering, all field combinations, edge cases)
• 2 pipeline tests verifying output survives ProcessText/filterSpecialChars
• Verified against real Slack email messages (with/without CC, with attachments, with inline images)