[plugin] Run plugin headless via electrum daemon/cli

Closes #41 (closed)

Runs the joinstr coinjoin flow headless via the Electrum daemon/CLI: a Qt-free JoinstrPlugin base class shared by qt.py and cmdline.py, with joinstr_create_pool / joinstr_join_pool / joinstr_list_pools / joinstr_my_pools commands. Tor and VPN transports, aut-ct sybil resistance, and Phase 3/4 DoS recovery all work headless.

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• electrum 4.6 cannot hold p2tr UTXOs (pubkey_to_address('p2tr') raises), so the aut-ct proving key can't come from a wallet UTXO; create_pool/join_pool accept an optional proving_key (p2tr WIF) passed straight to the autct API.
• aut-ct verify hits an upstream curve-tree root-parity limitation for keysets whose tree root has odd parity (the prove side and the joinstr gating logic are fine).