feat(subscribers): slim listing, API warnings, and filter

[imjohnbo]

Part 5 of 8. Base: #17.

What this adds

Slim listing:

kit subscribers list --slim

Filter:

kit subscribers filter --json '[{"type":"subscriber_state","states":["active"]}]'
kit subscribers filter --file conditions.json --include tags,stats --stats-start 2026-05-01

Request body changes

The API now documents two things about POST /v4/subscribers and PUT /v4/subscribers/{id}.

1. Custom field keys are the field key, not the label. Use last_name, not Last Name.
2. The API ignores an unknown key and reports it in a warnings array.

Create and update now print those warnings. A typo in --fields no longer passes silently. The --state help text lists all five states that the API accepts, and the CLI validates the value. The list filter separately accepts all, which is not a real state.

Design notes

The filter body nests too deeply for one flag per condition. It therefore comes from a file or an inline string. The CLI wraps a bare conditions array in the all object that the API wants. --counting-mode, --include, and the stats date range get their own flags.

Two related fixes ride along:

• subscribers create and subscribers update now take --format, like the rest of the CLI.
• printSuccess stays quiet under --format json. Piping into jq now works. Call sites that pass no options behave as before.

Open question for the reviewer

The spec declares no query parameters on POST /v4/subscribers/filter. Its response returns pagination cursors. This branch therefore sends per_page, after, and before as query parameters. If Kit expects them in the body, the fix is one line in src/commands/subscribers.js.

Review notes

Tests: 264 before, 306 after.

Closes #8
Closes #9
Closes #13

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The stack

Merge from the top down. Each part builds on the one above it.

	PR	Contents
1	#14	Client query support and shared helpers
2	#15	Sequence CRUD and sequence emails
3	#16	Posts and snippets
4	#17	Broadcast filters, account-wide stats, link clicks
5	#18 (this one)	Subscriber slim listing, warnings, filter
6	#19	Tag rename, remove-by-email, bulk delete
7	#20	Account endpoints and purchase create
8	#21	Spec coverage map and its tests

Base branch: main for part 1, the previous part's branch for the rest.
GitHub retargets each base to main as the part below it merges.