fix(ux): show helpful error messages on missing positional args

[natikgadzhi]

Summary

• Adds exactlyOneArg() helper that replaces cobra.ExactArgs(1) across 8 commands, producing a human-readable error with usage and examples when the user forgets the required positional arg
• Changes bare users (no subcommand) to show help/subcommand list instead of silently running users list
• Keeps rootCmd.SilenceUsage = true so runtime errors (auth failures, API errors) still don't dump usage

Before

$ slack-cli channels get
Error: accepts 1 arg(s), received 0

After

$ slack-cli channels get
slack-cli channels get requires a channel name or ID

Usage:
  slack-cli channels get <name|id> [flags]

Examples:
  slack-cli channels get general --since 2d
  slack-cli channels get C12345678

Commands updated

channels get, channels search, channels members, users get, users search, files read, emojis search, canvases read

Test plan

• go build ./... clean
• go vet ./... clean
• go test ./... all pass (including new TestExactlyOneArg_* tests)
• Manually verified each command prints the correct message with 0 args
• Manually verified "too many args" case (channels get foo bar) prints concise error
• Verified bare users shows help with subcommand list

🤖 Generated with Claude Code