fix(cli): keep a rejected --concurrency value off stdout

[ethanj]

Fixes #191, reported by @graysoncooper with a diagnosis that pointed straight at the cause.

What

quickstart --json promises a single parseable envelope on stdout. It did not deliver one when --concurrency was invalid:

$ llmwiki quickstart src.md --json --no-open --concurrency eight
! --concurrency value "eight" is not a positive integer; ignoring it.
{
  "version": 1,
  ...

Exit code is 0, so an automated caller sees success and then fails to parse.

Why it happened

The value is parsed while the CLI assembles the command's arguments, at cli.ts:389, and quiet mode is enabled later inside the command body at quickstart.ts:147. Argument expressions are evaluated before the callee runs, so quiet mode was never early enough to suppress it. Moving the setQuiet call earlier cannot fix this, because there is no point inside the command that precedes its own arguments.

The channel was the real problem. stdout is the data channel, and a complaint about invalid input belongs on stderr. output.note already exists for exactly that and has eighteen callers, so the fix is to use it.

That also fixes compile, refresh and watch, which take the same flag, and it keeps the warning visible rather than trading a corrupt envelope for a silent one.

Tests

• test/quickstart-json-stdout.test.ts drives the real CLI through dist/ and asserts both halves of the contract: stdout parses whole, and stderr still names the rejected value. No credentials needed, since the provider failure is reported inside the envelope, which is the shape a caller has to read anyway.
• The existing unit test asserted the rejection appeared on stdout, so it encoded the defect. It now pins the channel split instead.

Both tests were mutation-tested against the committed fix. Restoring the stdout write turns both red, and deleting the diagnostic altogether also turns both red, so a fix that simply silenced the warning would not pass.

The two other warnings in concurrency.ts are left alone deliberately: they run inside the command, where quiet mode governs them correctly.