Overseer: wire durable GitHub-side open-issue dedup on the gap-filing path (#4687 P2)

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Summary

The Overseer gap-scan documentation (PR #4710) originally described a restart-safe, GitHub-side open-issue dedup on the workstream-gap filing path:

WhisperGate.peek(sig) → gh.search_issues(repo, sig) → find_existing → reuse/skip/create

That flow is not yet implemented. The shipped code path (act_flag_workstream_gaps, src/overseer/mod.rs) uses only the in-process WhisperGate (gap_gate, 900 s window) + a consolidated operator notification (email + Signal). It never queries GitHub and never calls create_issue on the gap path, so its dedup guarantee is within a single running daemon and is wiped on restart.

PR #4710's docs were corrected (Step 18b) to describe the actual in-process behavior and to mark the GitHub-side durable check as future work. This issue tracks implementing it.

What already exists (the foundation this builds on)

• Bounded GapCategory taxonomy (GoalUncovered/IssueUncovered/AnomalyUnaddressed) — src/overseer/signal.rs.
• Stable, content-addressed dedup key GapItem::dedup_key() = workstream-gap:<signature> — restart-stable as a key.
• Injection-safe slug validation is_bounded_signature_slug / has_valid_dedup_signature (IV-1).
• The sibling stewardship::process_orchestrator_run + find_existing flow this should mirror.

Scope (P2 of #4687)

Wire the durable GitHub-side dedup into act_flag_workstream_gaps:

1. After the WhisperGate pre-filter, call gh.search_issues(repo, dedup_key) for an already-open equivalent issue.
2. find_existing on the stewardship-signature: body marker → reuse/skip if present.
3. Otherwise create_issue embedding stewardship-signature: <dedup_key>.
4. Fail-loud: a gh search error propagates and files nothing (never a blind create).
5. Add hermetic tests mirroring src/stewardship/tests_extra.rs.

Acceptance

• A daemon restart does not re-file an already-open gap issue (cross-process guarantee).
• overseer::gap_scan emits reused_existing when an open issue is matched.
• gh search failure files nothing and surfaces a first-class error.