fix: isolate subagent implicit resume chains

[icebear0828]

Summary

• Add local variant identity to implicit-resume variant hashes using Codex window id and task anchor signals.
• Keep upstream prompt_cache_key unchanged while isolating local previous_response_id chains and WS pool slots.
• Add regression coverage for single-thread multi-turn, interleaved conversations, derived-key conversations, same-shape subagents, and identical subagents separated by Codex window id.
• Stabilize a usage-stats numeric range test that sat exactly on the 30-day cutoff.

Verification

• npx tsc --noEmit
• git diff --check
• npm test (166 files passed, 1932 tests passed, 1 skipped)