fix(cli): config --help example generated from the live schema, rot-proofed by test

[plombeer31]

What

The config --help example was hand-written long ago and no longer runs:
it says "version":1 (supported versions start at 5, so config set rejects it
outright) and configures a llama key that was renamed to localModels back in
config v5. Anyone pasting the official example got a validation error.

The example is now assembled at module load from the live schema constants —
USER_CONFIG_VERSION and a localModels.url override — so it can never say a
version the parser refuses. The help also states the actual set semantics: keys
left out of the payload are filled with defaults, and the result is validated
before anything is written.

Rot-proofing

The new test does not check the example against a copy of itself — it extracts the
payload straight out of the rendered help text with a regex and feeds it through
the real config set path against a temp state dir, then asserts the written
file carries the values the example promises. If the schema moves and the help is
left behind again, this test fails instead of a user's paste.

Verified the test fails against the previous help text (no extractable valid
payload) and passes with the generated one.

Testing

• npm run lint (tsc) clean
• npx vitest run src/cli/config-command.test.ts — 9/9 (8 existing + the new rot-proofing test)