Refactor oversized merged modules and stabilize tests

[ScriptedAlchemy]

Summary

• split large inline test/helper blocks out of recently merged modules
• extract git-correlation attribution/backfill and tool-hint classifiers
• stabilize full lib tests by avoiding env races and slow daemon/socket E2Es
• document verified Cargo/libtest multi-filter behavior in project memory

Verification

• cargo fmt --check
• cargo test -q --lib
• cargo test -q --bin tracedecay tool_command::tests
• cargo test -q --test agent_suite
• cargo test -q --test mcp_suite sessions_for_and_scoped_search_end_to_end

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