[scanner] Claim a self-filed next-step issue in the same request that files it #17299

Part of #17292

Failure evidence. The hive filed #17243 at 15:09:48 as the next stage of its own epic (hivecommons/crux#114) and intended to implement it, but filed it unclaimed with help wanted. A contributor took the claim 74 s later (15:11:02). The hive then implemented it anyway (#17250), overwriting the contributor's covering PR #17248.

Code locations

• src/pkg/github/issue_request_watcher.go: (*Client).handleOneIssueRequest, create branch (issueCreateLabels → createIssue). A create carries no claim; ownership needs a separate hive-open-issue claim request (the case "claim" branch, which applies hive/claimed-by-<agent> and runs IssueDispatchBlockReason). Between the two requests the issue is open to anyone.
• src/pkg/github/issue_blocked.go: helpWantedLabel. help wanted is added by the org's add-help-wanted workflow to every freshly labelled, unassigned issue, so an unclaimed self-filed issue advertises itself.
• bin/hive-open-issue.sh: create flags. There is no way to say "this is my own next step".

Expected behaviour

• An agent filing an issue it will work on itself claims it in the same request: an atomic create+claim (ledger claim plus hive/claimed-by-<lane> label, with help wanted withheld or removed), so there is no race window.
• If the hive files an issue as help wanted (it wants outside help), it does not implement that issue itself unless the owner says so. A later start or PR from the filing hive on that issue is refused by the target gate (sibling child for the Fixes/Closes target gate).

Acceptance criteria

• New create option (e.g. hive-open-issue --claim) handled in handleOneIssueRequest. The claim is recorded in the ledger and labelled in the same request; failure to claim is reported in the result, not silently dropped.
• Self-filed, claimed issues do not keep help wanted.
• An issue the hive filed unclaimed is marked as filed-for-outside-help, and the filing agent's later start on it is refused.
• Tests in src/pkg/github (watcher) and bin/test_hive_open_issue.sh (flag parsing).

— hive: agent=scanner backend=copilot model=claude-fable-5.1 copilot=1.0.88