feat(pipeline): skip extraction when the source has not changed (#105)

Ingest now stamps source_hash — sha256 of the ingested body, frontmatter
excluded — onto the source page, and compares it before doing any work. An
unchanged source returns Skipped with no LLM call at all.

This is cost and signal, not correctness: #103 already made a re-ingest
byte-identical, so a repeated run was harmless but not free — a full
extraction over every chunk to produce a result that merges to nothing.
Building this first would have put idempotency in the guard rather than in
the write, which is the wrong dependency direction.

Body-only hashing on purpose. A re-crawl that stamps a new fetch date has
not changed the source, and treating that as drift would re-extract the
whole corpus every time a crawler ran. A missing or unreadable hash always
means "extract": a missed skip costs one extraction, a wrong skip loses an
update silently, and silence is the failure mode this repo keeps paying
for.

The skip is reported (Result.Skipped plus a warning naming the hash), not
silent — a run that does nothing must say so, or "the pipeline is fine" and
"the pipeline stopped working" look identical (#107).

Storing the hash required settling who owns which frontmatter field. #103
made the on-disk block win wholesale, which protects human-set metadata
from a regenerating writer but froze fields the generator legitimately
owns. wiki.Merge now refreshes an explicit generatorOwned allowlist —
last_updated and source_hash — and keeps everything else on disk. An
allowlist rather than a carve-out, so a field nobody has thought about
stays human-owned by default. That also retires the last_updated
limitation #103 shipped with.

Refs #105, #103, #107, brain#21.

ingestion/internal/wiki/merge.go

@@ -38,7 +38,7 @@ type section struct {
 // not present in a are appended.
 func Merge(a, b Page) Page {
 	fmA, preambleA, secsA := parseSections(a.Content)
-	_, _, secsB := parseSections(b.Content)
+	fmB, _, secsB := parseSections(b.Content)
 
 	idx := make(map[string]int, len(secsA))
 	for i, s := range secsA {
@@ -61,7 +61,56 @@ func Merge(a, b Page) Page {
 		}
 	}
 
-	return Page{Path: a.Path, Content: rebuildContent(fmA, preambleA, secsA)}
+	return Page{Path: a.Path, Content: rebuildContent(refreshOwned(fmA, fmB), preambleA, secsA)}
+}
+
+// generatorOwned names the frontmatter fields the ingest pipeline may refresh
+// on a page it is revising. Everything else on disk wins.
+//
+// #103 made the on-disk frontmatter win wholesale, which is what stops a
+// regenerating writer from overwriting human-set metadata — a title someone
+// fixed, or the claim-property envelope (brain#21). The cost was that fields
+// the generator legitimately owns froze at their first-ingest values: when it
+// last ran, and what it last saw.
+//
+// An explicit allowlist rather than a carve-out: the ownership boundary is
+// nameable, and a field nobody has thought about stays human-owned by default,
+// which is the safe direction.
+var generatorOwned = map[string]bool{
+	"last_updated": true,
+	"source_hash":  true,
+}
+
+// refreshOwned returns a's frontmatter with the generator-owned fields taken
+// from b where b supplies them. Fields absent from b keep a's value, so a
+// generator that stops emitting a field cannot blank it.
+func refreshOwned(a, b string) string {
+	bVals := make(map[string]string)
+	for _, line := range strings.Split(b, "\n") {
+		key, value, found := strings.Cut(line, ":")
+		if !found || !generatorOwned[strings.TrimSpace(key)] {
+			continue
+		}
+		bVals[strings.TrimSpace(key)] = strings.TrimSpace(value)
+	}
+	if len(bVals) == 0 {
+		return a
+	}
+
+	out := make([]string, 0, len(strings.Split(a, "\n")))
+	for _, line := range strings.Split(a, "\n") {
+		key, _, found := strings.Cut(line, ":")
+		k := strings.TrimSpace(key)
+		if found && generatorOwned[k] {
+			if v, ok := bVals[k]; ok {
+				out = append(out, k+": "+v)
+				delete(bVals, k)
+				continue
+			}
+		}
+		out = append(out, line)
+	}
+	return strings.Join(out, "\n")
 }
 
 // parseSections splits markdown into its frontmatter, its preamble (content