feat(garmin): implement data-fetch methods via generic call dispatch

GetActivities/GetActivitySplits/GetActivityDetails/GetWorkoutByID now send
{"cmd":"call","params":{"method":...,"args":...}} instead of named MCP
tools. subprocessClient fully implements Client.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/internal/garmin/client.go

@@ -12,6 +12,7 @@ import (
 	"io"
 	"os"
 	"os/exec"
+	"strconv"
 	"sync"
 )
 
@@ -311,3 +312,127 @@ func truncate(s string, n int) string {
 	}
 	return s[:n] + "...(truncated)"
 }
+
+var _ Client = (*subprocessClient)(nil)
+
+// NewClient builds a Client. The subprocess is not spawned until the first
+// call that needs it (Authenticate, or any data call once authenticated).
+func NewClient(cfg Config) Client {
+	return &subprocessClient{cfg: cfg}
+}
+
+func (c *subprocessClient) GetActivities(ctx context.Context, startDate, endDate string, limit int) ([]Activity, error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	if err := c.ensureStarted(); err != nil {
+		return nil, err
+	}
+	raw, err := c.roundTrip("call", callParams{
+		Method: "get_activities_by_date",
+		Args:   map[string]any{"startdate": startDate, "enddate": endDate},
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	var rawActivities []json.RawMessage
+	if err := json.Unmarshal(raw, &rawActivities); err != nil {
+		return nil, fmt.Errorf("get_activities_by_date did not return a JSON array (%q): %w", truncate(string(raw), 200), err)
+	}
+	if limit > 0 && limit < len(rawActivities) {
+		rawActivities = rawActivities[:limit]
+	}
+
+	activities := make([]Activity, 0, len(rawActivities))
+	for _, raw := range rawActivities {
+		var a Activity
+		if err := json.Unmarshal(raw, &a); err != nil {
+			return nil, fmt.Errorf("parse activity: %w", err)
+		}
+		a.Raw = raw
+		activities = append(activities, a)
+	}
+	return activities, nil
+}
+
+func (c *subprocessClient) GetActivitySplits(ctx context.Context, activityID int64) (ActivitySplits, error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	if err := c.ensureStarted(); err != nil {
+		return ActivitySplits{}, err
+	}
+	raw, err := c.roundTrip("call", callParams{
+		Method: "get_activity_splits",
+		Args:   map[string]any{"activity_id": strconv.FormatInt(activityID, 10)},
+	})
+	if err != nil {
+		return ActivitySplits{}, err
+	}
+
+	var envelope struct {
+		ActivityID int64             `json:"activityId"`
+		Laps       []json.RawMessage `json:"lapDTOs"`
+	}
+	if err := json.Unmarshal(raw, &envelope); err != nil {
+		return ActivitySplits{}, fmt.Errorf("get_activity_splits did not return valid JSON (%q): %w", truncate(string(raw), 200), err)
+	}
+
+	splits := ActivitySplits{ActivityID: envelope.ActivityID, Laps: make([]Lap, 0, len(envelope.Laps))}
+	for _, raw := range envelope.Laps {
+		var l Lap
+		if err := json.Unmarshal(raw, &l); err != nil {
+			return ActivitySplits{}, fmt.Errorf("parse lap: %w", err)
+		}
+		l.Raw = raw
+		splits.Laps = append(splits.Laps, l)
+	}
+	return splits, nil
+}
+
+func (c *subprocessClient) GetActivityDetails(ctx context.Context, activityID int64) (ActivityDetails, error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	if err := c.ensureStarted(); err != nil {
+		return ActivityDetails{}, err
+	}
+	raw, err := c.roundTrip("call", callParams{
+		Method: "get_activity_details",
+		Args:   map[string]any{"activity_id": strconv.FormatInt(activityID, 10)},
+	})
+	if err != nil {
+		return ActivityDetails{}, err
+	}
+
+	var details ActivityDetails
+	if err := json.Unmarshal(raw, &details); err != nil {
+		return ActivityDetails{}, fmt.Errorf("get_activity_details did not return valid JSON (%q): %w", truncate(string(raw), 200), err)
+	}
+	details.Raw = json.RawMessage(raw)
+	return details, nil
+}
+
+func (c *subprocessClient) GetWorkoutByID(ctx context.Context, workoutID int64) (Workout, error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	if err := c.ensureStarted(); err != nil {
+		return Workout{}, err
+	}
+	raw, err := c.roundTrip("call", callParams{
+		Method: "get_workout_by_id",
+		Args:   map[string]any{"workout_id": strconv.FormatInt(workoutID, 10)},
+	})
+	if err != nil {
+		return Workout{}, err
+	}
+
+	var workout Workout
+	if err := json.Unmarshal(raw, &workout); err != nil {
+		return Workout{}, fmt.Errorf("get_workout_by_id did not return valid JSON (%q): %w", truncate(string(raw), 200), err)
+	}
+	workout.Raw = json.RawMessage(raw)
+	return workout, nil
+}