feat(check): add --strict so a failed check exits non-zero

[Bccorb]

Refs #166

#166 asks to "decide the seamless check exit-code question, here or in a linked issue". #192 took the other branch it offers (the smoke job asserts independently), so that job does not depend on this. This is the decision, sent separately because it is a user-facing CLI flag rather than a CI change, and it touches all thirteen check sites.

The problem

From the issue, and reproduced:

$ seamless check          # in an empty directory
seamless.config.json not found
Run: seamless init
exit=0

$ seamless check          # scaffolded project, stack down
API not reachable
Auth not reachable
Console not reachable
Check complete.
exit=0

An empty directory, a stack that is down, and a fully healthy project are indistinguishable to anything reading the exit status. check is exactly the command you would put in a health-check script, and it cannot be used in one.

The fix

--strict exits 1 when any check failed, naming the count:

$ seamless check --strict
...
3 checks failed.
exit=1

Two decisions worth stating:

The default is unchanged. Anything parsing this output today keeps working, and a sudden non-zero exit would break it silently. That is why this is a flag rather than new default behaviour, as the issue suggests.

Every check still runs. Results go through a small Report that counts failures instead of throwing, so --strict decides the exit status only after the full report has printed. A gate that stops at the first problem hides the rest, and the whole picture is the reason to run check at all. There is a test for this: a project failing eight ways prints all eight and reports 8 checks failed.

The refactor

All thirteen pass/fail sites moved from bare console.log to report.ok(...) / report.fail(message, remedy?). That is what makes the count possible, and it also collapses the repeated two-line "failure then remedy" pattern into one call.

runCheck takes args so index.ts can pass the flag through; it defaults to [], so existing callers and tests are unaffected.

Verification

npm run build and npm test pass, 916 tests. 5 new tests. Also checked against the real binary rather than only through mocks:

no flag, empty dir:  exit=0
--strict, empty dir: exit=1

helpTopics.ts documents the flag, per the convention in AGENTS.md that the help registry is the single source. README does not mention check at all, so there is nothing to update there.