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

[TheGreatAxios]

Summary

• Quickstart previously showed a standalone script hitting a hardcoded local Ollama URL with a literal model name — not how a host actually calls this package.
• First revision invented an env-var-then-stored-credential fallback (resolveCredentialByName + CredentialCipher.decrypt) that no consumer in Workbench or @corbits/memory actually uses; that chain was removed from Workbench upstream, and @corbits/memory is env-only.
• Now shows the honest shape: an exported function taking a host-supplied EmbedConfig (baseURL/model/optional apiKey) and deps (host fetch + createDefaultScheduler()), calling embedTexts, plus probeEmbedDims at startup. Where the config comes from is explicitly the host's choice — the README points at @corbits/memory's EMBED_BASE_URL/EMBED_MODEL/EMBED_API_KEY env vars as one real example, without claiming this package itself resolves credentials.
• Trimmed the "## Interchange" section, which overstated adoption ("Interchange hubs and ingestion workers use this package") — no hub or worker imports @corbits/embedding today. It now only states what's actually true: this package shares @intx/inference's transport, error taxonomy, and retry behavior with chat calls.
• Renamed "Transport exports" to "Lower-level: transport" to mark those as secondary, standalone-useful exports rather than the primary entry point.
• Reverted the @intx/db devDependency added in the previous revision — no longer needed since the fence doesn't touch DB/credential types.

Test plan

• bunx tsc -p against a scratch file with the exact Quickstart code fence (deleted before commit) — compiles clean
• bun run typecheck (package's own tsc --noEmit) — passes
• bunx prettier -w README.md — applied