document one CLI rejection shape and exit 4

[indykish]

Pairs with agentsfleet/agentsfleet#617.

The published exit-code table in cli/flags.mdx already said 4 meant invalid command input. The client only did that for the handful of commands that validated their own flags — everything else exited 2, which the same table assigns to network failure. Now that the client agrees, cli/agentsfleet.mdx stops listing invalid input under 2; that row is network failure alone.

Adds the rejection output shape, the --json error envelope with its stable codes, and the group-help-on-stdout behavior.

🤖 Generated with Claude Code

Greptile Summary

The PR documents standardized CLI rejection handling, machine-readable error codes, exit code 4 for invalid invocations, and command-group help behavior.

• Updates the CLI exit-code reference so exit code 2 is reserved for network failures and rate limits.
• Adds human-readable and JSON rejection examples plus stable rejection codes.
• Records the behavior change in the changelog and documents group help on standard output.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
changelog.mdx	Adds release notes for standardized CLI rejection output, exit codes, JSON errors, and group help behavior.
cli/agentsfleet.mdx	Revises the command-reference error table and documents rejection output and stable JSON codes.
cli/flags.mdx	Documents exit code 4 for invalid input and standard-output help for bare command groups.

_{Reviews (2): Last reviewed commit: "docs(m171): the rejection sentence cover..." | Re-trigger Greptile}