feat: fxt 2.0 skeleton — new command tree with 15 command groups
Clean rewrite of the fxt CLI with unified gRPC client, JSON/human
output abstraction, and stub implementations for all 15 command groups:
auth, trade, train, tune, model, agent, backtest, broker, data,
service, cluster, risk, config, watch (TUI), mcp.
Deleted old fat-client commands, client modules, types, prelude, and
tests (-12,769 net lines). Fixed pre-existing clippy issues in auth
module (indexing_slicing in encryption.rs, manual_let_else in
token_manager.rs, str_to_string in build.rs).
60 tests passing (53 lib + 7 binary), 0 clippy warnings.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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