idle.ts eviction: upstream uses per-function cleanup in idle.ts, not centralized evictSession in pipeline.ts

Upstream (main branch) puts session eviction logic directly in idle.ts rather than a centralized evictSession() in pipeline.ts. idle.ts imports cleanup functions individually: evictSession as evictGradientSession from @loreai/core; also deleteSessionAuth, clearAuthStale from ./auth; deleteSessionCosts from ./cost-tracker; deleteBillingPrefix from ./cch; clearWarmupAuthDisabled from ./cache-warmer. The startIdleScheduler signature uses onEvict?: (sessionID: string) => void (upstream) vs onEvictSession?: (sessionID: string) => boolean (branch). Upstream inline onEvict in pipeline.ts cleans 5 Maps: headerSessionIndex, ltmSessionCache, ltmPinnedText, stableLtmCache, cwdWarned. When merging, adopt upstream's per-function approach and add any missing cleanup calls.