#63 FTX-63: MCP tool schema + parity fixes

proposed_action/context as type:object (no JSON-string double-encode); hitl_act_on_task gains reason_code/reason_text/justification (reject via MCP currently impossible); exceptions -> isError results; unknown tool -> error result; list_tools permission-filtered (UB-MCP-4); policy_allowlist (UB-MCP-8); add hitl_list_reason_codes + hitl_submit_context_update; handlers call domain services not route functions. AC-11 parity test. Phase 5.