feat: accept message body via --file or stdin for send/edit

[jonathanlam]

Problem

message send / message edit only accept the message body as a positional shell argument. For formatted Slack mrkdwn this is a footgun: in a double-quoted shell argument the shell performs backtick command substitution and $/quote expansion before slack-cli ever sees the text.

A real example that bit an agent: a body containing (`smithrx`) was sent as a double-quoted argument. The shell ran smithrx as a command (smithrx: command not found to stderr), the substitution evaluated to empty, and the message went out as SmithRx () — corrupted but with a successful ts, so the failure was silent.

Single-quoting only half-fixes it: single quotes can't contain a literal ', which Slack alerts routinely include.

Fix

Add a --file <path> flag (- means stdin) plus implicit piped-stdin support to both send and edit. Body resolution precedence:

1. --file (- = stdin)
2. positional text argument (unchanged — fully backward compatible)
3. piped stdin when no text argument is given

A quoted heredoc now sends arbitrary formatted bodies with zero shell interpretation:

slack-cli message send C01234ABC - <<'EOF'
:rotating_light: *Alert* with `code`, $vars, and 'quotes' all kept literal
EOF

Interactive stdin (a TTY) with no body returns a clear error instead of hanging.

Testing

• go build ./... + go vet ./... clean
• Verified manually against the built binary:
  • no body + no pipe → clean no message body error (no hang)
  • heredoc body with backticks / 'quotes' / $HOME → passes body resolution untouched, only fails at the (fake) auth call
  • --file <bad path> → clear read error
  • positional arg → still works (backward compat)
  • edit --file / stdin → works

Docs

SLACK_CLI_USAGE.md updated with the hazard note and --file/stdin usage for both commands.

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