Miscellaneous performance optimizations

[ChihweiLHBird]

Summary

Five small, low-risk hot-path cleanups (one commit each), no behavior change for the CLI:

• telemetry: cache the OTel logs enablement check (OnceLock) instead of re-reading env vars on every guest log write
• trigger-http: resolve the app name once at server construction instead of deserializing metadata on every request
• key-value-spin: exists uses SELECT 1 … LIMIT 1 instead of fetching the whole value BLOB via get
• llm-remote-http: move embedding vectors out of the response instead of cloning them
• llm-local: clone only eos_token_id instead of the whole llama Config

Notes

• Malformed name metadata now fails at server construction instead of per request (already caught earlier by the default factor stack, so unobservable in practice).
• SQLite exists no longer decodes the value, matching the other backends' native existence probes.