feat(llm): usage hook to surface token counts (ADR-030, #15)

WithUsageHook callback fires with model + prompt/completion tokens parsed from the
response usage block. Keeps the copied stdlib-only llm package decoupled from
metrics (ADR-004) — the caller wires it to internal/metrics. TDD covered.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/adapters/llm/client.go

@@ -32,6 +32,7 @@ type Client struct {
 	model      string
 	maxTokens  int
 	httpClient *http.Client
+	usageHook  func(model string, prompt, completion int)
 }
 
 // Option configures a Client at construction. Variadic so the existing 4-arg
@@ -50,6 +51,14 @@ func WithMaxTokens(n int) Option {
 	}
 }
 
+// WithUsageHook registers a callback fired after a successful completion with the
+// model and the prompt/completion token counts from the response usage block. It
+// keeps this copied, stdlib-only package (ADR-004) decoupled from metrics: the
+// caller wires it to internal/metrics, the client imports nothing. nil is ignored.
+func WithUsageHook(fn func(model string, prompt, completion int)) Option {
+	return func(c *Client) { c.usageHook = fn }
+}
+
 // New constructs a Client.
 func New(baseURL, apiKey, model string, timeout time.Duration, opts ...Option) *Client {
 	c := &Client{
@@ -81,6 +90,10 @@ type chatResponse struct {
 	Choices []struct {
 		Message message `json:"message"`
 	} `json:"choices"`
+	Usage struct {
+		PromptTokens     int `json:"prompt_tokens"`
+		CompletionTokens int `json:"completion_tokens"`
+	} `json:"usage"`
 }
 
 // Complete sends a system + user message and returns the assistant's reply.
@@ -152,5 +165,8 @@ func (c *Client) Complete(ctx context.Context, system, user string) (string, err
 	if len(cr.Choices) == 0 {
 		return "", fmt.Errorf("LLM returned no choices")
 	}
+	if c.usageHook != nil {
+		c.usageHook(c.model, cr.Usage.PromptTokens, cr.Usage.CompletionTokens)
+	}
 	return cr.Choices[0].Message.Content, nil
 }