docs(readme): Quickstart shows real host usage (CL-8775)

[TheGreatAxios]

Summary

• Quickstart now leads with installMemory, an exported function that mounts createMemory on a real Hono<TenantEnv> app with a real GrantStore/ConditionRegistry, matching the memory mount in a production Interchange hub's apps/hub/src/server.ts.
• Adds the second real mount deployed agents use: mountWorkflowMemory for run-scoped routes, plus the migration step (runMemoryMigrations) before serving traffic.
• Documents the embed config precedence accurately: EMBED_BASE_URL/EMBED_MODEL is a required pair (both or neither), no credential-based fallback — config is env-only, resolved once per process.
• Removes the toy in-process example that used createFakeDocumentStore as if it were a real host's Quickstart path; the fake store is now only mentioned under Development as a test utility.
• Keeps a real (non-fake) lower-level in-process example, config-backed, for a trusted single-tenant caller like the resident distiller.

Test plan

• Copied each ts fence into a scratch readme-check.ts at the repo root with a scratch tsconfig including it, ran bunx tsc --noEmit -p tsconfig.readme-check.json — clean, no errors. Scratch files deleted before commit.
• bun run typecheck passes.
• npx prettier --write README.md applied (repo has no prettier config/script of its own; formatting matches the tool's defaults already used across the file).

Follow-up

• Migration example used process.env.DATABASE_URL!, a non-null assertion. Replaced with a small exported migrateMemory(databaseUrl: string) and a fail-fast check on DATABASE_URL before calling it.