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

README.md

@@ -54,7 +54,7 @@ By default the reference tool uses:
 
 ```text
 ~/.canned-prompts/catalog
-~/.canned-prompts/registry
+~/.canned-prompts/default    # the default registry
 ```
 
 Override them with: