fix(wiki): surface merge conflicts as textual-diff warnings (closes #104)

Textual-diff conflict detection only (semantic contradiction detection via brain-gardener Judge deferred, confirmed by Mathias, comment 12594). Includes canonicalBlock whitespace-run normalization fix for the disagreement comparison.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Co-authored-by: Mathias <mthbqv@gmail.com>

ingestion/internal/wiki/merge.go

@@ -37,10 +37,33 @@ type section struct {
 // Sections are merged by strategy: bullet sections union unique lines,
 // append sections concatenate, all others keep a's version. Sections in b
 // not present in a are appended.
+// Merge discards any disagreement silently; prefer MergeReport where the
+// caller can act on it.
 func Merge(a, b Page) Page {
+	page, _ := MergeReport(a, b)
+	return page
+}
+
+// MergeReport is Merge, plus the headings of sections where b carried a
+// different version of a section a already had — and lost.
+//
+// That loss is the property which makes ingest safe to run unattended (#103):
+// a regenerating writer can add to a note but never rewrite it. But the
+// discarded text is also a signal, and until now it went in the bin: the source
+// now says something different about a section we already hold. Nobody could
+// tell "the source is unchanged" from "the source changed its mind and we
+// ignored it".
+//
+// Deliberately NOT a semantic contradiction check. This reports textual
+// disagreement, which is cheap, deterministic, and needs no model on the write
+// path. Deciding whether a disagreement is a genuine contradiction stays with
+// the Judge-gated enrich pass that already exists (brain-gardener#175), which
+// is approval-gated by design — detection here, adjudication there.
+func MergeReport(a, b Page) (Page, []string) {
 	fmA, preambleA, secsA := parseSections(a.Content)
 	fmB, _, secsB := parseSections(b.Content)
 
+	var conflicts []string
 	idx := make(map[string]int, len(secsA))
 	for i, s := range secsA {
 		idx[s.heading] = i
@@ -59,10 +82,15 @@ func Merge(a, b Page) Page {
 			secsA[i].content = mergeBullets(sA.content, sB.content)
 		case appendSections[sB.heading]:
 			secsA[i].content = appendNew(sA.content, sB.content)
+		default:
+			// a wins — but say that b disagreed, rather than dropping it mute.
+			if canonicalBlock(sA.content) != canonicalBlock(sB.content) {
+				conflicts = append(conflicts, sB.heading)
+			}
 		}
 	}
 
-	return Page{Path: a.Path, Content: rebuildContent(refreshOwned(fmA, fmB), preambleA, secsA)}
+	return Page{Path: a.Path, Content: rebuildContent(refreshOwned(fmA, fmB), preambleA, secsA)}, conflicts
 }
 
 // generatorOwned names the frontmatter fields the ingest pipeline may refresh
@@ -301,6 +329,21 @@ func canonicalLine(line string) string {
 	return strings.Join(strings.Fields(line), " ")
 }
 
+// canonicalBlock applies canonicalLine per line of a multi-line section body
+// — the same whitespace-insensitivity appendNew/mergeBullets already give a
+// single line, extended to a whole section for MergeReport's disagreement
+// check (found in review, PR #129: comparing raw strings.TrimSpace only
+// tolerated leading/trailing whitespace, not a re-extraction's mid-line
+// spacing, which every other comparison in this file already tolerates).
+func canonicalBlock(block string) string {
+	lines := strings.Split(block, "\n")
+	out := make([]string, len(lines))
+	for i, l := range lines {
+		out[i] = canonicalLine(l)
+	}
+	return strings.Join(out, "\n")
+}
+
 // appendNew concatenates the lines of b that a does not already carry.
 //
 // Append sections used to concatenate wholesale, which is correct exactly once.

ingestion/internal/wiki/merge_test.go

@@ -332,3 +332,85 @@ func TestRefreshOwned_IndentedListItemsWithColonsAreNotMisparsedAsKeys(t *testin
 	got := refreshOwned(a, b)
 	assert.Equal(t, "---\naliases:\n  - 'Chapter 3: The Reckoning'\ntitle: 'X'\nsource_hash: bbbb\n---\n", got)
 }
+
+func TestMerge_ReportsSectionsWhereTheFreshVersionWasDiscarded(t *testing.T) {
+	// The asymmetry that makes ingest safe (on-disk wins) also throws away a
+	// real signal: the source now says something different about a section we
+	// already have. Today that disagreement is silently discarded, so nobody
+	// can tell "the source is unchanged" from "the source changed its mind and
+	// we ignored it".
+	existing := Page{Path: "wiki/sources/x.md", Content: `---
+title: 'X'
+---
+
+## Summary
+
+Appetite is fixed, scope is variable.
+
+## Method
+
+Six-week cycles.
+`}
+	fresh := Page{Path: "wiki/sources/x.md", Content: `---
+title: 'X'
+---
+
+## Summary
+
+Appetite is negotiable and scope is fixed.
+
+## Method
+
+Six-week cycles.
+`}
+
+	got, conflicts := MergeReport(existing, fresh)
+
+	assert.Contains(t, got.Content, "Appetite is fixed", "on-disk version still wins")
+	assert.NotContains(t, got.Content, "negotiable")
+	assert.Equal(t, []string{"Summary"}, conflicts,
+		"only the section that actually disagreed is reported")
+}
+
+func TestMerge_ReportsNothingWhenTheFreshVersionAgrees(t *testing.T) {
+	page := Page{Path: "wiki/sources/x.md", Content: `---
+title: 'X'
+---
+
+## Summary
+
+Appetite is fixed, scope is variable.
+`}
+
+	_, conflicts := MergeReport(page, page)
+	assert.Empty(t, conflicts, "identical content is not a disagreement")
+}
+
+func TestMerge_ReportsNothingForInternalWhitespaceVariantsOnly(t *testing.T) {
+	// Found in review of PR #129: the conflict comparison used raw
+	// strings.TrimSpace, which only tolerates LEADING/TRAILING whitespace —
+	// unlike appendNew/mergeBullets, which already canonicalize internal
+	// whitespace runs too (#103: a re-extraction's spacing isn't guaranteed
+	// byte-stable). A mid-line double-space from re-extraction would
+	// false-positive as a "disagreement" here even though every other
+	// whitespace-sensitive comparison in this file already tolerates it.
+	existing := Page{Path: "wiki/sources/x.md", Content: `---
+title: 'X'
+---
+
+## Summary
+
+Appetite  is fixed,   scope is variable.
+`}
+	fresh := Page{Path: "wiki/sources/x.md", Content: `---
+title: 'X'
+---
+
+## Summary
+
+Appetite is fixed, scope is variable.
+`}
+
+	_, conflicts := MergeReport(existing, fresh)
+	assert.Empty(t, conflicts, "internal whitespace differences alone are not a disagreement")
+}