feat(web): landing Memory Playground + ?format=markdown API

[harrymove-ctrl]

What

A Firecrawl / context.dev-style Memory Playground on the landing page, plus a small backend addition so the memory API can return Markdown directly.

Frontend — apps/web

• Instant, zero-auth playground with four agent-native verbs: recall · ask · knowledge · remember, each calling the public Worker live.
• Markdown ⇄ JSON response tabs; cURL / TS / Python snippet tabs with copy-to-clipboard.
• Snippets are pinned to the public Worker origin (not the build-time API base), so a copied request runs for any external dev.
• Honest source labeling: anonymous recall is shown as "Curated facts · keyword-ranked," not "live Walrus semantic," because a no-delegate caller only ever gets the keyword-ranked seeded facts.

Backend — apps/api/src/worker.ts

• ?format=markdown (alias ?format=md) on POST /api/memwal/recall, POST /api/memwal/chat, and GET /api/memwal/facts/:ns.
• Lets agents consume recall hits / grounded answers / namespace knowledge as Markdown instead of parsing the JSON envelope. Mirrors the playground's client-side rendering for consistency.
• Query-param keyed only (no Accept-header Vary), so cached facts responses can't be poisoned across formats. Default JSON behavior is unchanged when the param is absent.

Verification

• bun run typecheck — clean across all 7 workspaces.
• Worker wrangler deploy --dry-run — 3.8 MB bundle (no jsdom leak), all new symbols present.
• Prod web build — no localhost baked into the bundle.
• Playground verified live in-browser: recall renders Markdown + curated-facts badge; ask does grounded RAG and correctly declines out-of-namespace questions.

Deliberately out of scope

The write path (remember) is gated to a CTA, not wired — recall reads from the MemWal relayer while a write would Seal-encrypt to Harbor (different backends, no round-trip), and the only recallable write needs a 64-char delegate. Left for a follow-up that reconciles the read/write backend split.