feat(ingestion): lint and repair unparseable note frontmatter (#90)

Hand-authored and promoted notes skip pipeline.BuildPages, so nothing
quotes their scalars. A title carrying an unescaped ": " yields a block
yaml.Unmarshal rejects, and every consumer that reads frontmatter drops
the note silently — 18 files repo-wide today, up from the 8 in #90.

frontmatter.Lint reports it; frontmatter.Fix quotes only what quoting can
rescue and refuses to return a document that still fails to parse, so an
unfixable file comes back byte-identical rather than half-repaired. Scan
walks a tree and reports an unreadable file as a finding instead of
aborting, so one poisoned note cannot hide the rest.

Sequence items are deliberately left alone: "- foo: bar" is valid YAML
that silently becomes a map, which is a type corruption rather than a
parse failure, and repairing it would rewrite notes that currently load.

    task lint:frontmatter          # report
    task lint:frontmatter FIX=1    # repair in place

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BeAp5LTscnz2W7Ubt4eJ6m

ingestion/cmd/frontmatterlint/main.go

@@ -0,0 +1,53 @@
+// Command frontmatterlint reports markdown notes whose YAML frontmatter does
+// not parse, and optionally repairs the ones that only need quoting.
+//
+//	go run ./cmd/frontmatterlint ../../brain          # report
+//	go run ./cmd/frontmatterlint -fix ../../brain     # repair in place
+//
+// Exit codes: 0 clean (or everything repaired), 1 findings remain, 2 the scan
+// itself failed.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/frontmatter"
+)
+
+func main() {
+	fix := flag.Bool("fix", false, "repair files that quoting can rescue, in place")
+	flag.Parse()
+
+	root := "."
+	if flag.NArg() > 0 {
+		root = flag.Arg(0)
+	}
+
+	findings, err := frontmatter.Scan(root, *fix)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "frontmatterlint:", err)
+		os.Exit(2)
+	}
+
+	var unresolved int
+	for _, f := range findings {
+		if f.Repaired {
+			fmt.Printf("repaired  %s\n", f.Path)
+			continue
+		}
+		unresolved++
+		fmt.Printf("BROKEN    %s: %v\n", f.Path, f.Err)
+	}
+
+	if unresolved > 0 {
+		fmt.Fprintf(os.Stderr, "\n%d file(s) with unparseable frontmatter", unresolved)
+		if !*fix {
+			fmt.Fprint(os.Stderr, " — re-run with -fix")
+		}
+		fmt.Fprintln(os.Stderr)
+		os.Exit(1)
+	}
+	fmt.Printf("✓ frontmatter: %d file(s) repaired, none broken\n", len(findings))
+}