--help leaves out .mts configs, DESIGN.md and inspect --format json #222

What happens

onsystem --help is behind the code in three places:

• --config lists onsystem.config.{json,ts,mjs,js}, but onsystem.config.mts is read too.
• --tokens says "DTCG .json or CSS", but a DESIGN.md is a token file too.
• --format lists values for check and export only. inspect --format json works and is tested, but isn't listed, and inspect --format xml (or a typo) exits 0 with the text output, where check and export exit 2 on an unknown format.

Where

• src/cli.ts:79, src/cli.ts:83 and src/cli.ts:87-88: the help lines
• src/config.ts:294-300: CONFIG_FILES, which includes onsystem.config.mts
• src/tokens/index.ts:69: .md token files
• src/cli.ts:1107-1122: inspect, which only checks for json (and inspectWorkspace at :1263 the same)
• src/cli.ts:357-361: how check rejects an unknown format
• test/cli.test.ts:115-121 and :479-483: the usage-error and help tests

Done when

• The help names .mts in the --config default, DESIGN.md in --tokens, and inspect: pretty | json under --format
• inspect --format xml exits 2 with one line in the style of check's, at a monorepo root too
• A test checks that --help names every extension in CONFIG_FILES, and the usage-error test covers inspect --format xml
• pnpm test, pnpm lint and pnpm typecheck pass; a patch changeset

Hint

Check the format at the top of inspect() (src/cli.ts:1107), before loadTarget, so it covers the monorepo path too. CONFIG_FILES is exported from src/config.ts. #202 (in review) changes inspect's output a few lines further down: if it merges first, rebase onto it.

Out of scope

docs/configuration.md, which #202 also changes.

Status: ready for a contributor. Comment before you start.