Document local-dev models setup matching production

[heskew]

The last unwritten piece of HarperFast/harper#510's documentation acceptance ("Documented: writing a custom backend, configuring fallback, local-dev setup matching production"):

• Writing a custom backend — done (docs(models): document registerBackend / defineBackend custom backends #554, corrected in docs(models): namespace registration API + document routing & fallback #558).
• Configuring fallback — done (docs(models): namespace registration API + document routing & fallback #558 added reference/models/routing.md, documenting the [Models] Pluggable model routing + multi-backend addressing (resolves #510 fallback decision) harper#1326 conclusion: fallback is a router concern layered on the routing seam — fallback groups, capability routing, custom routers — not a core facade behavior).
• Local-dev setup matching production — this issue.

Scope: a page under reference/models/ covering:

• The portability model — application code addresses models by logical name; the local ↔ production swap is configuration-only.
• A local configuration with the ollama backend (no credentials) and its matching production configuration (hosted providers + ${VAR} credential indirection).
• Parity caveats: the ollama backend does not advertise tools (workaround: openai backend + baseUrl against an OpenAI-compatible local server such as vLLM); embedding vectors are not comparable across different models — matters when @embed data or HNSW indexes move between environments.
• Per-environment configuration: per-instance config files; HARPER_CONFIG merge for containerized deployments.
• Offline / CI stubs via models.registerBackend / models.defineBackend (a registered backend shadows a same-name config entry).

Part of #548 (v5.1 coverage umbrella). Docs deliverable for HarperFast/harper#510.