feat(brain): validate the claim-property envelope on write (brain#21)

First half of brain#21: the applicability envelope — half_life (volatile|
seasonal|durable), applies (local|homelab|universal), and claim_provenance
(asserted|inferred). The strength half (confidence, contested) follows once
the ingest-time contradiction detector exists to populate it.

writeHallNote already passed unknown frontmatter keys through verbatim, so
preservation was free — a test now pins it so a refactor of the merge loop
cannot silently drop the fields. What was missing is validation, and it
fails closed:

- a value outside the enum is rejected. An unvalidated value is worse than
  an absent one, because a lint skips what it cannot parse and the note then
  reads as classified while nothing acts on it;
- an envelope field without claim_provenance is rejected. A value a model
  inferred must never be indistinguishable from one Mathias asserted —
  otherwise a single classifier pass launders machine guesses into apparent
  human judgement across 1113 notes, which is precisely how a weak claim
  hardens into accepted fact;
- provenance without any envelope field is rejected too: it says who set
  fields that are not there.

Validation runs before MkdirAll, so a rejected write leaves nothing behind —
not even an empty hall directory.

The field is `applies`, not `scope`, as brain#19 proposed: `scope:` has been
in use since brain#5 for glossary terms (homelab-core, project:<repo>), a
disjoint value space on the same key. The shipped schema keeps the name.

Notes with none of these fields are byte-identical to before. Bulk backfill
across the corpus stays out of scope — the same failure mode as the
extraction pass that produced 28 wrong wikilinks out of 67.

Schema documented in brain/CLAUDE.md (via brain_edit_root, which produced
the #97 audit record for the rewrite).

Refs brain#21, brain#19 (superseded), brain#5, hyperguild#97.

ingestion/internal/api/handler.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"github.com/mathiasbq/hyperguild/ingestion/internal/brain"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/claim"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/extract"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/pipeline"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/search"
@@ -152,15 +153,24 @@ func writeHallNote(brainDir string, opts WriteNoteOptions) (string, error) {
 	if err != nil {
 		return "", err
 	}
-	if err := os.MkdirAll(filepath.Dir(dest), 0o755); err != nil {
-		return "", fmt.Errorf("create hall dir: %w", err)
-	}
-
 	existingFields, body := splitFrontmatter(opts.Content)
 	existingByKey := make(map[string]frontmatterField, len(existingFields))
 	for _, f := range existingFields {
 		existingByKey[f.key] = f
 	}
+
+	// Validate the claim-property envelope BEFORE creating the directory, so a
+	// rejected write leaves nothing behind — not even an empty hall dir
+	// (brain#21). Fail closed: a value outside the enum is worse than an absent
+	// one, because a lint skips what it cannot parse and the note then reads as
+	// classified while nothing acts on it.
+	if err := claim.ValidateEnvelope(scalarValues(existingFields)); err != nil {
+		return "", fmt.Errorf("claim envelope: %w", err)
+	}
+
+	if err := os.MkdirAll(filepath.Dir(dest), 0o755); err != nil {
+		return "", fmt.Errorf("create hall dir: %w", err)
+	}
 	emitted := make(map[string]bool, 6)
 
 	var fm strings.Builder
@@ -218,6 +228,25 @@ func writeHallNote(brainDir string, opts WriteNoteOptions) (string, error) {
 	return filepath.ToSlash(rel), nil
 }
 
+// scalarValues flattens frontmatter fields to key -> scalar value for
+// validation. Multi-line values (lists, nested blocks) yield the text after
+// the colon on the first line, which is empty for a list — correct here,
+// since every claim-property field is a scalar enum.
+func scalarValues(fields []frontmatterField) map[string]string {
+	out := make(map[string]string, len(fields))
+	for _, f := range fields {
+		if len(f.lines) == 0 {
+			continue
+		}
+		_, value, found := strings.Cut(f.lines[0], ":")
+		if !found {
+			continue
+		}
+		out[f.key] = strings.TrimSpace(value)
+	}
+	return out
+}
+
 // frontmatterField is one top-level YAML key from a frontmatter block,
 // along with its raw line and any indented continuation lines (e.g. a
 // bulleted list value spanning multiple lines).