fix(ownership): report a Cloudflare challenge as its own error and result

When Cloudflare answers idx.co.id with a challenge, curl fails with only
`curl: (22)`, which exits 4 like any other HTTP failure, and every profile
is tried twice. A challenge needs a browser, so no profile can solve it. The
source is monthly, so the publisher only needs one run per month to get
through: treat a challenge as "retry tomorrow" and alert on stale data.

- Add IdxError::CloudflareChallenge (CLOUDFLARECHALLENGE, exit 6), detected
  from the cf-mitigated header. curl reports it on stderr through --write-out,
  because --fail drops the response.
- A URL with a challenge gets one round of profiles; other failures keep two.
  This also covers `stocks flow --source idx`, which now exits 6 on a challenge.
- -v logs which profile fetched each URL.
- Discovery keeps error types, so a challenge survives an announcement miss.
  A source answering with something unparseable still is not called a challenge.
- `ownership discover --require-data-page` fails when the data page cannot be
  read. Default discover is unchanged: it warns and lists older announcements.
- The publisher and builder use that flag, so a failed data page can no longer
  end in a false "up-to-date". A challenge ends in `RESULT: blocked stage=<stage>`,
  exit 6; every other failure stays `RESULT: FAILED`.
- The systemd example treats exit 6 as success and runs the freshness check, so
  the unit fails on FAILED or on stale data.
- live-smoke reports a challenge as CHALLENGED, a failure and never a pass.

Golden change: completions-bash.golden gains `--require-data-page` in the
discover options, one word on one line. It is the direct result of the new flag;
no other golden file changes.

Live check on 2026-11-05 against release/v0.5.0: 19 passed, 2 failed. Both
failures are real challenges (flow-idx, discover), each exit 6 with empty stdout.
The import, releases and ticker cases did not run, because discover did not pass.

Refs #83

web/src/content/docs/agents/index.md

@@ -20,7 +20,7 @@ What an agent needs to know:
   `{"error": true, "code": "...", "message": "..."}`.
 - Exit codes say what to do next: `2` fix the input, `3` not found (or no
   cached data offline), `4` provider unavailable, retry later, `5`
-  unsupported, `1` other.
+  unsupported, `6` Cloudflare challenge from idx.co.id, retry later, `1` other.
 - Tickers are IDX codes (`BBCA`); `.JK` is optional. Several tickers can be
   passed at once to `stocks quote` and `stocks compare`.
 - Ownership queries need `idx ownership sync` once; after that they run