feat(ai): AmbientDrift — frame-grounded drift judge

internal/ai/ambient.go

@@ -0,0 +1,79 @@
+// internal/ai/ambient.go
+package ai
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"strings"
+)
+
+// AmbientDriftJudge judges whether current activity serves the user's standing
+// frame when NO task is declared. Like Nudge it takes primitives, not domain
+// types, so ai stays a leaf. The returned string is a one-sentence advisory, or
+// "" when the activity is plausibly on-track.
+type AmbientDriftJudge interface {
+	AmbientDrift(ctx context.Context, frame string, recentTitles []string) (string, error)
+}
+
+// ErrInvalidAmbientDrift marks a parseable-but-unusable ambient response.
+var ErrInvalidAmbientDrift = errors.New("ai: invalid ambient drift")
+
+// AmbientDrift makes Service satisfy AmbientDriftJudge over the same backend.
+func (s *Service) AmbientDrift(ctx context.Context, frame string, recentTitles []string) (string, error) {
+	out, err := s.backend.Run(ctx, buildAmbientPrompt(frame, recentTitles))
+	if err != nil {
+		return "", err
+	}
+	return parseAmbientDrift(out)
+}
+
+func buildAmbientPrompt(frame string, recentTitles []string) string {
+	return `You are an ambient coach. The user has NOT declared a task. Below is their standing frame — goals, values, and life-domains — and today's tasks, then the recent sequence of window titles. Decide whether the recent activity plausibly serves ANY of what matters to them, or whether it is a slide into drift.
+
+Be forgiving: legitimate breaks, rest, admin, and unplanned-but-useful work are ON-TRACK. Only call drift when the activity clearly serves none of what matters.
+
+Respond with ONLY a JSON object, no prose and no code fences, exactly this shape:
+{"drifting": <true or false>, "message": "<short explanation, one sentence>"}
+
+Rules:
+- drifting: true only if the recent activity serves none of the frame; false otherwise.
+- message: one short sentence naming the drift. REQUIRED when drifting is true.
+
+## Frame
+` + frame + `
+
+## Recent window titles (oldest to newest)
+` + strings.Join(recentTitles, "\n")
+}
+
+type rawAmbientDrift struct {
+	Drifting bool   `json:"drifting"`
+	Message  string `json:"message"`
+}
+
+// parseAmbientDrift extracts the advisory from raw CLI output. On-track yields
+// "". A drift with no message degrades to "" (silence) rather than an error:
+// the ambient signal is advisory, so the safe degenerate is to say nothing —
+// exactly like parseNudge.
+func parseAmbientDrift(s string) (string, error) {
+	if strings.TrimSpace(s) == "" {
+		return "", ErrEmptyResponse
+	}
+	if strings.IndexByte(s, '{') < 0 {
+		return "", ErrNoJSON
+	}
+	jsonStr, err := extractJSON(s)
+	if err != nil {
+		return "", fmt.Errorf("%w: %v", ErrInvalidAmbientDrift, err)
+	}
+	var raw rawAmbientDrift
+	if err := json.Unmarshal([]byte(jsonStr), &raw); err != nil {
+		return "", fmt.Errorf("%w: %v", ErrInvalidAmbientDrift, err)
+	}
+	if !raw.Drifting {
+		return "", nil
+	}
+	return strings.TrimSpace(raw.Message), nil
+}