feat: pre-populate commit dialog with heuristic message from git diff

Generate a commit message from modified files (e.g. "Update winter-2026"
or "Update 12 notes") and pre-fill the CommitDialog textarea so users can
commit immediately or edit the suggestion.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/utils/commitMessage.ts

@@ -0,0 +1,33 @@
+import type { ModifiedFile } from '../types'
+
+const VERB_MAP: Record<string, string> = {
+  modified: 'Update',
+  added: 'Add',
+  untracked: 'Add',
+  deleted: 'Delete',
+  renamed: 'Rename',
+}
+
+const MAX_LISTED_FILES = 3
+
+function noteName(relativePath: string): string {
+  const basename = relativePath.split('/').pop() ?? relativePath
+  return basename.replace(/\.md$/, '')
+}
+
+function verb(files: ModifiedFile[]): string {
+  const statuses = new Set(files.map((f) => f.status))
+  if (statuses.size === 1) return VERB_MAP[files[0].status] ?? 'Update'
+  return 'Update'
+}
+
+/** Generate a heuristic commit message from modified files. */
+export function generateCommitMessage(files: ModifiedFile[]): string {
+  if (files.length === 0) return ''
+  const action = verb(files)
+  if (files.length <= MAX_LISTED_FILES) {
+    const names = files.map((f) => noteName(f.relativePath)).join(', ')
+    return `${action} ${names}`
+  }
+  return `${action} ${files.length} notes`
+}