docs(v0.4.1): add nested git guidance for project mode

Project CLAUDE.md template now includes a Git section stating that the
workspace uses a single git repo at the root and subdirectory git init
is not permitted. docs/commands.md picks up the same guidance under the
--project flag section.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/seed/templates.go

@@ -62,6 +62,12 @@ This is a ctask project workspace -- a long-lived working environment, not a dis
 - Record important assumptions and actions in notes.md.
 - Keep the workspace root clean.
 
+## Git
+
+This workspace uses a single git repository at the workspace root.
+Do not initialize additional git repositories inside subdirectories.
+All project code, even if nested in a subdirectory, is tracked by the root repo.
+
 ## Session Handoff
 
 Before ending a session, append a brief summary to notes.md with:

internal/seed/templates_test.go

@@ -62,6 +62,18 @@ func TestClaudeMDProjectContainsRequiredSections(t *testing.T) {
 	}
 }
 
+func TestClaudeMDProjectContainsNestedGitRule(t *testing.T) {
+	body := ClaudeMDProject()
+	for _, must := range []string{
+		"single git repository",
+		"Do not initialize additional git repositories",
+	} {
+		if !strings.Contains(body, must) {
+			t.Errorf("ClaudeMDProject missing guidance %q", must)
+		}
+	}
+}
+
 func TestNotesMDIsASCII(t *testing.T) {
 	content := NotesMD("test title")
 	for i, b := range []byte(content) {