--update-mask always fails with 400: the v4 PATCH request message has no updateMask field

--update-mask is documented on every writable type's update subcommand, but the v4 API has no such field on the PATCH request message, so passing the flag always fails with a 400.

Reproduction

Straight from the CLI, no custom tooling, on weight (already Writable: true with update support):

$ ghealth data weight create --json '{"weight":{"weightGrams":1234,"notes":"repro - delete me","sampleTime":{"physicalTime":"2026-09-03T18:00:00Z","utcOffset":"-18000s"}}}'
# -> created id 1919002790956405426

$ ghealth data weight update --id 1919002790956405426 \
    --json '{"weight":{"weightGrams":4321}}' \
    --update-mask weight.weightGrams
{
  "error": {
    "type": "api",
    "code": 1,
    "status": 400,
    "message": "Invalid JSON payload received. Unknown name \"updateMask\": Cannot bind query parameter. Field 'updateMask' could not be found in request message."
  }
}

$ ghealth data weight update --id 1919002790956405426 \
    --json '{"weight":{"weightGrams":4321,"sampleTime":{"physicalTime":"2026-09-03T18:00:00Z","utcOffset":"-18000s"}}}'
# -> 200 OK, weightGrams now 4321

Same result with snake_case (?update_mask=...) sent directly to the API, so it isn't a casing mismatch in cmd/data.go:

400 Invalid JSON payload received. Unknown name "update_mask": Cannot bind query parameter.

Test data was deleted afterwards and confirmed gone (404).

Cause

newUpdateCommand in cmd/data.go sets the mask as a query parameter:

if updateMask != "" {
    req.Query = url.Values{"updateMask": {updateMask}}
}

The API rejects it because users.dataTypes.dataPoints.patch has no updateMask in its request message — the v4 discovery document lists no query parameters for that method at all. So the flag can only ever turn a working request into a 400.

Suggested fix

Either drop --update-mask, or keep it and fail fast client-side with a clear message rather than sending a request that is guaranteed to 400. Worth a line in the README's write section too, since the current wording implies partial updates are supported — PATCH appears to be whole-object, which matters because a partial body silently drops the fields you leave out.

Happy to send a PR for whichever you prefer.

Environment

• ghealth built from main @ 9cf0274, go build -o ghealth .
• go1.26.0 linux/amd64

Start in cmd/data.go at newUpdateCommand and reproduce the documented weight update command to confirm the API's 400 response. Check the v4 discovery behavior described in the issue, then implement the chosen handling for --update-mask and update the README write section if applicable; done means the flag no longer sends a guaranteed-invalid request and whole-object PATCH behavior is documented.