Problem

Slack silently truncates messages exceeding 40,000 characters. When slck messages send posts a message that's too long, Slack returns ok: true with a message_truncated warning in response metadata — but slck doesn't parse that warning, so it reports success. The caller has no idea their message was cut off mid-content.

Real-world example: An AI agent used slck messages send to post a ~45K character questionnaire into a Slack thread. The message was truncated mid-word ("Extensio...") and the agent never knew.

Solution

Add client-side message length validation to messages send and messages update:

1. Error by default when text exceeds 40,000 characters (Slack's hard limit)
2. Actionable error message with character count and alternatives:
   • --file <path> to upload as a file attachment
   • slck canvas create to create a Slack canvas
   • Manual chunking suggestion
3. Skip validation for file uploads (the --file path doesn't have this limit)

Scope

Code changes

• internal/cmd/messages/messages.go — maxMessageTextLen constant + validateMessageLength() function
• internal/cmd/messages/send.go — validation call in runSend
• internal/cmd/messages/update.go — validation call in runUpdate
• Tests for all of the above

Documentation

• CHANGELOG.md — true up: collapse [Unreleased] into versioned sections ([3.2.0] for new + post-v3.1.35 work, [3.1.35] for previously released items)
• README.md — add Message Length Limits entry under Known Limitations