fix(monitor): reprime re-added wallets

[cgfixit]

Summary

• Clear the primed marker for wallets removed by TradeMonitor.set_wallets().
• A wallet re-added after a rebalance now seeds a fresh activity baseline before it can emit trades.
• Add the regression test and mark DD-12 fixed in the active invariant ledger.

Why

Previously, a removed wallet retained its primed marker. If it returned later, its first poll could treat trades accumulated during the gap as new. The existing staleness gate is not a substitute for the cold-start guard.

Verification

• python -m pytest tests/test_monitor.py -q -p no:cacheprovider — 56 passed
• python -m pytest -m "not integration" -q -p no:cacheprovider — 625 passed
• powershell -NoProfile -ExecutionPolicy Bypass -File scripts/check-lint.ps1
• python -m mypy polymarket_copier --ignore-missing-imports --no-strict-optional
• python -m py_compile polymarket_copier/core/monitor.py tests/test_monitor.py
• git diff --check

Scope and risk

Monitor state only. No trading math, position sizing, order behavior, live-mode gate, or credentials changed. Retained wallets keep their existing priming state and seen IDs.