feat(ollama): add session persistence

[hugosmoreira]

Summary

• restore persisted user, model, and agent messages into Ollama chat history
• preserve the configured Ollama system prompt while replacing stale conversation state
• skip unsupported or empty session records consistently with the other providers
• add regression coverage for role conversion, invalid records, repeated initialization, and session clearing

Why

Ollama previously ignored persisted messages in Initialize, so loading or switching sessions could not restore the conversation. This converts stored API messages into Ollama messages while retaining the system prompt that Ollama keeps in its history.

Validation

• go test ./gollm -run '^TestOllama' -count=1
• go test -race ./gollm -run '^TestOllama' -count=1
• go test ./pkg/agent ./pkg/sessions -count=1
• go test ./... -count=1
• go test ./... -count=1 in kubectl-utils
• repository presubmits: build, vet, format, module tidiness, autogeneration, and mocks

Closes #464