chore(v2): retire V1 paths post-cutover

[smypmsa]

Context

Post-cutover cleanup. Open and execute after 2026-04-28 11:00 UTC once the V2 flip is verified live and stable. Keep the V1 paths in place for 1–2 weeks before retiring, in case a regression forces a temporary V1 fallback.

References:

• V2 Migration Guide
• Contract Addresses

Scope

• Remove py-clob-client from backend/pyproject.toml; keep only py-clob-client-v2
• Drop V1 entries from backend/core/wallet/contracts.py: USDC_E, V1 CTF_EXCHANGE, V1 NEG_RISK_CTF_EXCHANGE
• Remove the POLYMARKET_V2_ENABLED feature flag and all V1 branches across backend/core/trading/, backend/core/positions/, backend/core/wallet/
• Delete the V1 swap helper if not already replaced
• Verify experiments/onchain-otc/* still work — these use the NegRisk adapter directly (separate from CTF Exchange) and should be unaffected, but spot-check
• Update README.md / CLAUDE.md if any V1-specific guidance remains

Out of scope

• Behavior changes — strictly cleanup of dead code paths.

Timing

Open after cutover succeeds. Wait at least 1 week before merging to ensure no V2 regressions need a quick V1 fallback.

Prerequisites

• feat(v2-prep): parallel V2 stack — py-clob-client-v2, V2 contracts, pUSD config #27, feat(v2): collateral plumbing — USDC.e → pUSD #28, feat(v2): flip ClobClient to V2 — order struct, EIP-712 v2, match-time fees #36 all merged.
• Cutover (2026-04-28 ~11:00 UTC) executed and V2 verified live.
• At least 1 week of stable V2 operation before this lands, in case a regression forces a temporary V1 fallback.

Blocks

None — final cleanup in the series.