[codex] Add quiet target leases for benchmarks

[nathanwhit]

Summary

Adds benchmark quiet-mode support for worker sessions:

• adds target-scoped quiet leases backed by target_quiet locks
• makes scheduler placement skip quiet-reserved targets, including pinned requests
• exposes request_quiet_target / release_quiet_target MCP tools to workers and follow-ups
• sweeps stale quiet locks during recovery and periodic maintenance
• documents the timed-section workflow in worker prompts

Why

Benchmark work sometimes needs a genuinely quiet machine for accurate timing, but the fleet should still run multiple sessions per target outside those measurement windows. This makes quiet mode temporary and explicit instead of lowering target capacity permanently.

Validation

• go test ./internal/orch -run 'TestQuietTarget|TestMCPSurfaces' -count=1\n- go test ./internal/orch ./internal/store ./internal/model ./internal/api