feat(cli): point top-level cli --help at --format json

CLI_USAGE described outer --json but did not mention tool-level
--format json. Add a help-only pointer that tools which accept format
support --format tree|json, and distinguish payload JSON from the full
MCP envelope. No new flags, env vars, or output-default changes.

Fixes #2102

Signed-off-by: Lowpower <3133734+Lowpower@users.noreply.github.com>