User says "3-of-5" meaning 3 signers needed.
tss-lib threshold t means t+1 signers required.
Now we store t-1 at session creation (like persistent-only does).
Changes:
- co_managed_handler.go: tssThresholdT = req.ThresholdT - 1
- tss-party/main.go: remove -1 from sign (now consistent with keygen)
BREAKING: Existing co-managed wallets must be regenerated.
ROLLBACK: Revert this commit if signing still fails.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>