## Summary

Coding agents that run the CLI now get JSON output by default, so they
can parse structured results instead of scraping human-formatted tables.
I resolve the effective output format once at the TypeScript entrypoint
(both the legacy and next roots): an explicit `--output` /
`--output-format` always wins, otherwise a detected coding agent gets
`json` and everyone else keeps `text`. For Go-proxied commands this
flows down as `--output json` to the Go subprocess, which renders JSON
for the data-returning commands and ignores the flag elsewhere, so no Go
changes were needed.

## Changes

- Resolve output format from coding-agent detection in both CLI roots,
defaulting agents to `json` when no format is set explicitly.
- Add a shared `resolveAgentOutputFormat` helper (`explicit ?? (agent ?
json : text)`) reused by both roots, with unit tests.
- Make the `--output-format` global flag optional so an
explicitly-passed format is distinguishable from the default.
- Suppress the agent JSON default whenever the Go-compat `-o`/`--output`
flag is passed explicitly: `-o json|yaml|toml|env` already
short-circuits inside handlers, and `-o pretty` now keeps rendering the
human table on natively ported commands instead of being overridden to
JSON.

## Behavior notes

- Humans and CI keep their current output: production telemetry shows
~99.6% of CI invocations carry no coding-agent env signal, so they don't
flip.
- Error rendering follows the resolved format. In text mode errors stay
as red text on stderr, unchanged. In agent-default JSON mode handler
errors emit the existing structured envelope
(`{"_tag":"Error","error":{code,message}}`) on stdout with a non-zero
exit code, the same behavior `--output-format json` already had, so
agents get parseable failures instead of prose.
- Interactive prompts already refuse to block in JSON mode
(`NonInteractiveError`), so a detected agent cannot hang on a confirm
prompt.
- `--help` and `--version` render before the output layer engages and
stay text in both roots.
- `--agent yes|no` overrides detection in both directions for the
default-format decision.

## Linear

- fixes GROWTH-913

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Co-authored-by: Julien Goux <hi@jgoux.dev>