(cli): --json is a global option but is silently ignored by most commands

[iliapolo]

Describe the bug

--json (-j) is declared as a global CLI option, which makes it appear available on every cdk command in help output and causes it to be accepted (without error) on any command. In reality there is no generic mechanism that serializes command output as JSON. Only a handful of commands actually read the flag and opt in by hand:

• context
• list / ls
• bootstrap (via --show-template)
• synth / synthesize
• metadata

For every other command (deploy, diff, drift, destroy, import, gc, rollback, etc.) --json is accepted but silently ignored. This is misleading: users reasonably assume a global option affects all commands, so they may pass cdk deploy --json expecting machine-readable output and instead get ordinary output with no error and no indication the flag did nothing.

The flag's own description already hints that its scope is narrow — "Use JSON output instead of YAML when templates are printed to STDOUT" — yet it is registered globally.

Regression Issue

• Select this option if this issue appears to be a regression.

Last Known Working CDK Version

N/A

Expected Behavior

--json should only be available on the commands that actually honor it (context, list/ls, bootstrap, synth, metadata). Passing --json to a command that does not support it should either not be accepted (yargs reports an unknown option) or be clearly documented as unsupported — rather than being silently ignored.

Current Behavior

--json is a global option. Passing it to a command that does not read it (e.g. cdk deploy --json) is accepted silently and has no effect on the output.

Reproduction Steps

# Accepted, but has no effect — output is unchanged and no warning/error is shown:
cdk deploy --json

# Contrast with a command that actually honors it:
cdk ls --json

Possible Solution

Move --json from the global options block in packages/aws-cdk/lib/cli/cli-config.ts into the per-command option definitions for the commands that consume it (context, list/ls, bootstrap, synth/synthesize, metadata). Regenerate the argument parser via @aws-cdk/user-input-gen.

Note this is an observable CLI-surface change: commands that currently accept --json as a no-op would begin rejecting it as an unknown option. This likely warrants a feature flag or a deprecation window to preserve backwards compatibility, per the repo's backwards-compatibility policy.

Additional Information/Context

Discovered while adding a generic guard to the integration-test harness (fixture.cdk()) that asserts --json output is JSON-parseable. Because the flag is a no-op on most commands, the guard can only meaningfully be applied to the commands listed above.

CDK CLI Version

N/A (current main)

Framework Version

N/A

Node.js Version

N/A

OS

N/A

Language

N/A

Language Version

N/A

Other information

The five consumers were located in packages/aws-cdk/lib/cli/cli.ts (reads of argv.json / args.json); the global declaration is in packages/aws-cdk/lib/cli/cli-config.ts.