Remove github.com/tmc/langchaingo v0.1.13 and rewrite azure.go and
ollama.go using Go's standard library (net/http, encoding/json) — the
same secure pattern already used by huggingface.go.

- azure.go: direct REST calls to Azure OpenAI chat/completions and
  embeddings endpoints with api-key header auth
- ollama.go: direct REST calls to Ollama /api/generate and /api/embed
  using Ollama's native JSON API (no auth required)
- go.mod/go.sum: remove langchaingo and its exclusive transitive deps
  (dlclark/regexp2, pkoukk/tiktoken-go, pkg/errors), reducing the
  dependency attack surface

No interface changes — all callers of llm.Provider are unaffected.

https://claude.ai/code/session_012X8wf4jwQFRYfMnbbWCEs5