CANP-WP-0003: name the default registry default

Section 20.1 takes a registry's name from its directory basename, so the
reference tool's default store at ~/.canned-prompts/registry was named
`registry` — giving `registry:practice/thing` as a qualified reference.

The workplan had recorded this as reading poorly in one output line. It was
understated: the name reaches qualified references, the on-disk catalog layout
(catalog/registry/...), and index.yaml rows (registry: registry). Once packages
are installed and inclusions recorded, changing it becomes a migration rather
than an edit.

Renaming the directory to ~/.canned-prompts/default fixes it with no special
case in code, nothing written into the user's store, and no second naming
mechanism competing with section 20.1's manifest.

check_legacy_registry refuses to silently create a fresh empty store beside a
populated ~/.canned-prompts/registry — that failure would have been worse than
the wart being fixed. It names the exact mv, and the --registry flag for
keeping the old store. The check fires only for the default path; an explicit
--registry is never second-guessed.

Tests 95 -> 99.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Bjefh8NUiEiahN4JLwoSKM

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 388925@bnt-lap001
Assistant-Session: 3507023f-e0fd-4a1e-9d90-a0d4217d1502

reference/canned_prompts.py

@@ -55,9 +55,30 @@ def default_catalog() -> Path:
 
 
 def default_registry() -> Path:
+    # Named "default" rather than "registry" because § 20.1 takes a registry's
+    # name from its directory basename, and that name reaches qualified
+    # references, catalog paths and index rows.
+    return home_dir() / "default"
+
+
+def legacy_registry() -> Path:
     return home_dir() / "registry"
 
 
+def check_legacy_registry(registry: Path) -> None:
+    """Refuse to silently start a fresh default store beside a populated one."""
+    if registry.resolve() != default_registry().resolve():
+        return
+    if registry.exists() or not legacy_registry().is_dir():
+        return
+    raise CannedPromptError(
+        f"the default registry moved from {legacy_registry()} to {registry}, so "
+        f"a registry's basename names it (§ 20.1) rather than reading as "
+        f"'registry:'. Move it:\n    mv {legacy_registry()} {registry}\n"
+        f"or keep using the old one explicitly with --registry {legacy_registry()}"
+    )
+
+
 def read_manifest(package_dir: Path) -> dict[str, Any]:
     manifest_path = package_dir / "prompt.yaml"
     if not manifest_path.is_file():
@@ -858,6 +879,7 @@ def cmd_publish(args: argparse.Namespace) -> None:
     src = Path(args.path)
     manifest = validate_package(src)
     registry = Path(args.registry).expanduser()
+    check_legacy_registry(registry)
     name = registry_name(registry) if registry.is_dir() else None
 
     policy, claim = namespace_policy(registry, manifest["id"]) if registry.is_dir() else ("open", None)
@@ -879,6 +901,7 @@ def cmd_publish(args: argparse.Namespace) -> None:
 
 def cmd_install(args: argparse.Namespace) -> None:
     registry = Path(args.registry).expanduser()
+    check_legacy_registry(registry)
     catalog = Path(args.catalog).expanduser()
     name = registry_name(registry)