Add structural-duplicate gate before meta_argument move

Insert a pre-argument gate that computes a normalized shape-hash (title n-gram fingerprint + change_type + sorted target_files set) for each candidate entering the argument move. Before invoking the $0.11 LLM call, query hypotheses table for rows in the last 60d with status IN ('council_rejected','commander_killed','pending_commander') sharing the same shape-hash. If a match exists, short-circuit with kill reason 'structural_duplicate_pre_argument' and emit a move record at $0 cost. Reuse the existing 15ed71-detection logic already used in S148 backfill — promote it from a post-hoc cleanup into a pre-flight gate.

On the 5-day Phase 1 trace (7× v2_backfill_orphan + 1× structural_duplicate_15ed71), at least 2-3 of those kills migrate from post-argument to pre-argument, saving ~$0.20-0.33 in argument cost per Phase 1 week. Argument move cost drops measurably in the next cycle telemetry.

Replay the last 14 days of meta cycles through the new gate offline. If fewer than 2 candidates would have been caught pre-argument (i.e. duplicates only emerge post-argument because the title is rewritten by argument), the gate's hash position is wrong and it should move post-argument or be removed.

Rejected by filter_score. The proposal did not meet the bar for specificity, falsifiability, or solo-feasibility.