Bulk storyboard validation needs backoff or concurrency control

[bokelley]

Problem

Running all 21 storyboards sequentially against the training agent causes rate limit cascading. The creative_ad_server storyboard fails because earlier storyboards consume the rate limit budget.

The per-storyboard scenario filtering (#1985, #433) solved the single-storyboard case — each evaluation now runs 2-5 MCP calls instead of 30+. But bulk validation (running all storyboards against one agent) still accumulates calls and hits limits.

Current behavior:

• Each storyboard evaluation runs its own comply() call
• No delay between storyboards
• Agent rate limits (~20 calls) are hit by storyboard 5-6
• Later storyboards get 405/429 responses and cascade-fail

Options:

1. Sequential with backoff: Add delay between storyboard evaluations when running bulk validation
2. Concurrency limit: Run at most N storyboards concurrently
3. Shared session context: Deduplicate discovery calls (get_products, list_creative_formats) across storyboards that share the same agent
4. Agent-side rate limit headers: Respect Retry-After headers from the agent

Option 3 aligns with the long-term goal from #433 (option 4: deduplicate discovery).

Context

Discovered during storyboard validation (#1985). The creative_ad_server storyboard consistently fails when run after other storyboards but passes when run standalone.