fix(wiki): append generator-owned frontmatter fields a page never had, not just replace

Review of PR #126 found refreshOwned only replaced a generator-owned line
that already existed in a's frontmatter -- it never appended one a didn't
have. Every page ingested before this feature shipped has no source_hash
line at all, so on its first re-ingest after deploy, the fresh hash was
silently dropped: storedSourceHash reads "" forever for that page, the
hash never matches, and the skip-on-unchanged feature this whole PR/issue
exists to build never engages for the entire pre-existing corpus,
permanently, with nothing telling anyone.

Fixed by tracking the closing frontmatter fence's position while
scanning a's lines, then inserting any generator-owned fields b supplied
that a didn't have just before that fence -- sorted for a deterministic,
byte-stable result, since map iteration order isn't. Flip-tested:
forcing the old drop-silently return turns the new test red; restoring
the fix turns it green.

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

ingestion/internal/wiki/merge_test.go

@@ -214,3 +214,39 @@ Rewritten by the model.
 	assert.Contains(t, got, "Original.", "an existing section still wins over the model's rewrite")
 	assert.NotContains(t, got, "Renamed by the model")
 }
+
+func TestMerge_GeneratorOwnedFieldsAreAppendedWhenAPageNeverHadThemBefore(t *testing.T) {
+	// Found in review of PR #126: refreshOwned only replaced a generator-owned
+	// line that already existed in a's frontmatter — it never appended one a
+	// didn't have. Every page ingested before this feature shipped has no
+	// source_hash line at all, so on its first re-ingest after deploy, the
+	// fresh hash was silently dropped and the skip-on-unchanged feature this
+	// PR exists to build would never engage for the entire pre-existing
+	// corpus, permanently, with nothing telling anyone.
+	existing := Page{Path: "wiki/sources/x.md", Content: `---
+title: 'X'
+---
+
+## Summary
+
+Original.
+`}
+	fresh := Page{Path: "wiki/sources/x.md", Content: `---
+title: 'Renamed by the model'
+last_updated: 2026-08-22
+source_hash: bbbb
+---
+
+## Summary
+
+Rewritten by the model.
+`}
+
+	got := Merge(existing, fresh).Content
+
+	assert.Contains(t, got, "source_hash: bbbb", "a generator-owned field a page never had before must be appended, not dropped")
+	assert.Contains(t, got, "last_updated: 2026-08-22")
+	assert.Contains(t, got, "title: 'X'", "a human-set title must still survive regeneration")
+	assert.Contains(t, got, "Original.")
+	assert.NotContains(t, got, "Renamed by the model")
+}