llm: route every LLM caller through a shared backend client

Replaces the per-plugin Ollama HTTP calls with internal/llm, which picks a
backend from the environment (vLLM or Ollama) behind one Chat interface, plus
internal/plugin/llm_client.go as the plugin-facing wrapper.

Startup now logs llm_backend/llm_endpoint/llm_model instead of the two
OLLAMA_* vars, which no longer describe where inference actually goes.

These files were already running in prod from the vLLM migration but had never
been committed; this is that live state, byte-for-byte.

internal/llm/ollama.go

@@ -0,0 +1,103 @@
+package llm
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+)
+
+// OllamaClient talks to Ollama's native /api/generate endpoint.
+type OllamaClient struct {
+	backend
+}
+
+func (c *OllamaClient) Backend() string { return "ollama" }
+
+type ollamaOptions struct {
+	NumCtx      int     `json:"num_ctx,omitempty"`
+	NumPredict  int     `json:"num_predict,omitempty"`
+	Temperature float64 `json:"temperature,omitempty"`
+}
+
+type ollamaRequest struct {
+	Model   string        `json:"model"`
+	Prompt  string        `json:"prompt"`
+	System  string        `json:"system,omitempty"`
+	Stream  bool          `json:"stream"`
+	Think   bool          `json:"think"`
+	Options ollamaOptions `json:"options,omitempty"`
+}
+
+// Generate posts a single non-streaming generation and returns the completion.
+func (c *OllamaClient) Generate(ctx context.Context, req Request) (string, error) {
+	ctx, cancel := context.WithTimeout(ctx, c.timeoutFor(req))
+	defer cancel()
+
+	body, err := json.Marshal(ollamaRequest{
+		Model:  c.model,
+		Prompt: req.Prompt,
+		System: req.System,
+		Stream: false,
+		Think:  false,
+		Options: ollamaOptions{
+			NumCtx:      req.NumCtx,
+			NumPredict:  req.MaxTokens,
+			Temperature: req.Temperature,
+		},
+	})
+	if err != nil {
+		return "", fmt.Errorf("ollama: marshal payload: %w", err)
+	}
+
+	httpReq, err := http.NewRequestWithContext(ctx, http.MethodPost,
+		c.endpoint+"/api/generate", bytes.NewReader(body))
+	if err != nil {
+		return "", fmt.Errorf("ollama: build request: %w", err)
+	}
+	httpReq.Header.Set("Content-Type", "application/json")
+
+	resp, err := http.DefaultClient.Do(httpReq)
+	if err != nil {
+		return "", fmt.Errorf("ollama request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	respBody, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return "", fmt.Errorf("ollama: read response: %w", err)
+	}
+	if resp.StatusCode != http.StatusOK {
+		return "", fmt.Errorf("ollama HTTP %d: %s", resp.StatusCode, string(respBody))
+	}
+
+	var result struct {
+		Response string `json:"response"`
+	}
+	if err := json.Unmarshal(respBody, &result); err != nil {
+		return "", fmt.Errorf("ollama: parse response: %w", err)
+	}
+	return StripThink(result.Response), nil
+}
+
+// Ping lists locally installed models via Ollama's native /api/tags.
+func (c *OllamaClient) Ping(ctx context.Context) ([]string, error) {
+	ctx, cancel := context.WithTimeout(ctx, pingTimeout)
+	defer cancel()
+
+	var out struct {
+		Models []struct {
+			Name string `json:"name"`
+		} `json:"models"`
+	}
+	if err := getJSON(ctx, c.endpoint+"/api/tags", &out); err != nil {
+		return nil, err
+	}
+	names := make([]string, 0, len(out.Models))
+	for _, m := range out.Models {
+		names = append(names, m.Name)
+	}
+	return names, nil
+}