P5a: digests cite pages; sift_pdf_* metrics; sift fetch --pages/--grep; skill, unit and README

Digest systemPrompt gains a page-cite instruction and PromptVersion is 3.
Add the sift_pdf_* metric series (extract_total{outcome}, extract_seconds,
pages_total, pool_wait_seconds, cache_hits_total) to internal/metrics and wire
ObservePDF/ObservePDFWait/PDFCacheHit into internal/api/pdf.go. Refactor
cmd/sift/fetchcmd.go to a fetchOpts struct with parseFetchOpts and fetchBody,
adding --pages/--grep. Update skill/SKILL.md, set nix MemoryMax to 1G, and add a
PDF documents section to the README.

Co-Authored-By: ornith-1.5-35b-a3b <ornith-1.5-35b-a3b@llama.cpp.invalid>

internal/metrics/pdf_test.go

@@ -0,0 +1,49 @@
+package metrics
+
+// P5a task 07: the sift_pdf_* series.
+
+import (
+	"io"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+)
+
+type zeroSrc struct{}
+
+func (zeroSrc) Snapshot() Snapshot { return Snapshot{} }
+
+func TestPDFSeries(t *testing.T) {
+	s := New(zeroSrc{})
+	s.ObservePDF("ok", 1500*time.Millisecond, 212)
+	s.ObservePDF("ok", 200*time.Millisecond, 3)
+	s.ObservePDF("no_text", 50*time.Millisecond, 0)
+	s.ObservePDF("blocked: evil.example", time.Millisecond, 0) // raw text never becomes a label
+	s.ObservePDFWait(2 * time.Second)
+	s.PDFCacheHit()
+	s.PDFCacheHit()
+
+	req := httptest.NewRequest("GET", "/metrics", nil)
+	req.RemoteAddr, req.Host = "127.0.0.1:9", "127.0.0.1:8890"
+	rec := httptest.NewRecorder()
+	s.Handler().ServeHTTP(rec, req)
+	body, _ := io.ReadAll(rec.Body)
+	out := string(body)
+	for _, want := range []string{
+		`sift_pdf_extract_total{outcome="ok"} 2`,
+		`sift_pdf_extract_total{outcome="no_text"} 1`,
+		`sift_pdf_extract_total{outcome="other"} 1`,
+		`sift_pdf_pages_total 215`,
+		`sift_pdf_cache_hits_total 2`,
+		`sift_pdf_extract_seconds_count 4`,
+		`sift_pdf_pool_wait_seconds_count 1`,
+	} {
+		if !strings.Contains(out, want) {
+			t.Errorf("metrics lack %q", want)
+		}
+	}
+	if strings.Contains(out, "evil.example") {
+		t.Error("a raw outcome string leaked into a label")
+	}
+}