[contexto]: Configurable API base URL + timeout/retry in RemoteBackend

[sm86]

Problem

RemoteBackend (packages/contexto/src/client.ts):

• API_BASE is hardcoded — no way to point at staging/self-hosted/local.
• fetch() calls have no timeout and no retry. Transient 5xx during compact() silently drops the buffered episodes.

Goal

• Configurable base URL via apiBaseUrl config + CONTEXTO_API_BASE env var.
• Timeout (AbortController) on both ingest and search.
• Bounded retry with backoff on 5xx/network errors only.

Existing graceful-degrade behavior (no crash on failure) stays.