feat(brain): /pass-rate aggregator and handler

Adds a new HTTP GET handler at the ingestion pod that walks
brain/sessions/*.jsonl, filters by skill name and timestamp window
(default 7d, accepts Nh and Nd), normalizes legacy status vocabulary
(ok->pass, error->fail, skipped->skip), and returns aggregated counts
plus pass_rate.

Pass rate is null when pass+fail == 0, distinguishing 'no data' from
'always passes'. Plan 6 routing pod will check for null before
making decisions.

Route registration in cmd/server/main.go lands in a follow-up commit.

ingestion/internal/api/passrate.go

@@ -0,0 +1,140 @@
+package api
+
+import (
+	"encoding/json"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+)
+
+type passRateResponse struct {
+	Skill    string   `json:"skill"`
+	Window   string   `json:"window"`
+	Pass     int      `json:"pass"`
+	Fail     int      `json:"fail"`
+	Skip     int      `json:"skip"`
+	Total    int      `json:"total"`
+	PassRate *float64 `json:"pass_rate"`
+}
+
+// PassRate handles GET /pass-rate?skill=X&window=Y.
+// Walks brainDir/sessions/*.jsonl, filters by skill name and timestamp,
+// returns aggregated counts and pass rate.
+func (h *Handler) PassRate(w http.ResponseWriter, r *http.Request) {
+	skill := r.URL.Query().Get("skill")
+	if skill == "" {
+		writeError(w, http.StatusBadRequest, "skill is required")
+		return
+	}
+
+	windowStr := r.URL.Query().Get("window")
+	if windowStr == "" {
+		windowStr = "7d"
+	}
+	window, err := parseWindow(windowStr)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid window: "+err.Error())
+		return
+	}
+
+	cutoff := time.Now().UTC().Add(-window)
+	pass, fail, skip := 0, 0, 0
+
+	sessionsDir := filepath.Join(h.brainDir, "sessions")
+	entries, err := os.ReadDir(sessionsDir)
+	if err != nil && !os.IsNotExist(err) {
+		writeError(w, http.StatusInternalServerError, "read sessions dir: "+err.Error())
+		return
+	}
+
+	for _, entry := range entries {
+		if entry.IsDir() || !strings.HasSuffix(entry.Name(), ".jsonl") {
+			continue
+		}
+		body, err := os.ReadFile(filepath.Join(sessionsDir, entry.Name()))
+		if err != nil {
+			continue // skip unreadable files
+		}
+		for _, line := range strings.Split(string(body), "\n") {
+			line = strings.TrimSpace(line)
+			if line == "" {
+				continue
+			}
+			var rec struct {
+				Timestamp   string `json:"timestamp"`
+				Skill       string `json:"skill"`
+				FinalStatus string `json:"final_status"`
+			}
+			if err := json.Unmarshal([]byte(line), &rec); err != nil {
+				continue // malformed — skip
+			}
+			if rec.Skill != skill {
+				continue
+			}
+			ts, err := time.Parse(time.RFC3339, rec.Timestamp)
+			if err != nil {
+				continue
+			}
+			if ts.Before(cutoff) {
+				continue
+			}
+			switch normalizeStatus(rec.FinalStatus) {
+			case "pass":
+				pass++
+			case "fail":
+				fail++
+			case "skip":
+				skip++
+			}
+		}
+	}
+
+	total := pass + fail + skip
+	resp := passRateResponse{
+		Skill:  skill,
+		Window: windowStr,
+		Pass:   pass,
+		Fail:   fail,
+		Skip:   skip,
+		Total:  total,
+	}
+	if pass+fail > 0 {
+		rate := float64(pass) / float64(pass+fail)
+		resp.PassRate = &rate
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	_ = json.NewEncoder(w).Encode(resp)
+}
+
+// normalizeStatus maps both new (pass/fail/skip) and legacy (ok/error/skipped)
+// vocabularies to the canonical pass/fail/skip set. Unknown values are treated
+// as skip for safety.
+func normalizeStatus(s string) string {
+	switch s {
+	case "pass", "ok":
+		return "pass"
+	case "fail", "error":
+		return "fail"
+	case "skip", "skipped":
+		return "skip"
+	default:
+		return "skip"
+	}
+}
+
+// parseWindow accepts Go-style durations plus "Nd" for days.
+func parseWindow(s string) (time.Duration, error) {
+	if strings.HasSuffix(s, "d") {
+		// Replace "d" with "h" * 24
+		days := strings.TrimSuffix(s, "d")
+		d, err := time.ParseDuration(days + "h")
+		if err != nil {
+			return 0, err
+		}
+		return d * 24, nil
+	}
+	return time.ParseDuration(s)
+}