chore(output): say task, not to-do, in the --json error kind

[ryanlewis]

Closes #219

The --json error payload spelled one concept two ways. kind was "task" everywhere except the not a project failure, where it was "to-do", while type on a task row has said "task" since #214. One word wins, and that word is task.

What changed

ProjectEditCmd.Run now builds its wrongKindError with Kind: "task". That field feeds both the JSON kind and the rendered message, so the message moves with it: "Post letter" is a task; use things edit. Every other producer already said task, project, area or tag.

The docs that print or explain the payload follow: the error-payload example in the bundled skill, the token paragraph on the agents page, and the README paragraph explaining what kind carries. The skill gains a one-line note in its --json section saying a to-do is a task in every JSON value the CLI emits, naming import payloads as the one exception.

commands.md and configuration.md are untouched. Neither documents the error payload, and no command or flag changed.

Not a breaking change

kind: "to-do" was introduced in #193, which is not an ancestor of v0.7.0. The value has never appeared in a release, so no published contract moves and there is nothing for the 0.8.0 notes to warn about. That is the same reasoning #223 applied to the type rename. The error token is untouched either way: it stays not a project.

Out of scope, deliberately

import payloads still spell a to-do "to-do". That is Things' own JSON URL scheme format and it stays documented as the exception.

Three other kind variables hold "to-do" and were left alone, because none of them reaches JSON. They only compose English prose: the repeating-item refusal in verify.go, the per-item refusal line in importcheck.go, and the agent brief's opening sentence in internal/output/agent.go. The issue scopes itself to JSON values, and renaming these would rewrite prose and its tests well outside that. The new comment on the Kind field says so explicitly, so the next reader does not take the rule as wider than it is.

How tested

make test and make lint both green. TestRunProjectEditRefusesTodoReference asserts the new kind and message and carries a comment saying why. The built binary was run against the live Things database to confirm the payload it prints.