signals.py: tool_bucket helper + three tool_histogram-based extractors that the
outcome/marker signals were blind to — sig_infra_overhead (hub+task+schema share
of tool calls over threshold), sig_schema_thrash (repeated ToolSearch), and
sig_tool_thrash (one tool dominating). Thresholds in build_context. 8 new tests;
suite 88/88 green.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
