feat: add session recording, replay, and scenario filtering to eval harness

[BYK]

Summary

Adds session recording/replay to the eval harness for ~90% cost reduction on subsequent runs, plus scenario filtering for targeted testing.

New CLI flags

# First run: record session replay data
bun packages/core/eval/run.ts --mode live --record fixtures/recorded --dimensions preferences

# Subsequent runs: replay from fixtures (skip expensive upstream API calls)
bun packages/core/eval/run.ts --mode live --replay fixtures/recorded --dimensions preferences

# Run only specific scenarios
bun packages/core/eval/run.ts --mode live --scenarios pr-3-evolution --baselines lore

How it works

Session replay is the dominant eval cost (~$2-3 per run with 24+ upstream Anthropic API calls with growing context windows). The recording system:

1. Record mode (--record <dir>): Enables the gateway's existing startRecording() interceptor during replaySession() calls. Upstream request/response pairs are saved as NDJSON files (one per session). The interceptor is scoped to session replay only — /lore:curate and QA calls are unaffected.

2. Replay mode (--replay <dir>): Loads recorded NDJSON fixtures and wires the gateway's getReplayInterceptor() during session replay. The gateway still fully processes every turn (temporal storage, session tracking, gradient state) — only the upstream HTTP call is replaced. /lore:curate and QA questions still use real API calls.

Cost comparison (PR-3 evolution scenario)

Mode	Session replay	Distill + Curate	QA + Judge	Total
Record (first run)	~$2-3 (24 calls)	~$0.30	~$0.05	~$2.35
Replay (subsequent)	$0 (fixtures)	~$0.30	~$0.03	~$0.33

Other changes

• --scenarios filter: Run only specific scenarios by ID (comma-separated)
• Removed 5s QA delay: No longer needed with /lore:curate handling curation synchronously

Files Changed

• packages/core/eval/types.ts — recordDir, replayDir, scenarios fields on EvalConfig
• packages/core/eval/run.ts — CLI flags + validation
• packages/core/eval/harness.ts — replaySessionWithFixtures(), scenario filtering, delay removal