[codex] hide plugin command from root slash menu

[nightt5879]

Summary

• Fix /plugin discovery tiering by using the canonical command name in the Advanced list.
• Expose discovery tier name lists to registry tests so aliases cannot drift into the hardcoded tier list again.
• Cover the slash menu acceptance: /plugin is hidden from bare / discovery but appears when typing /pl.

Fixes #3910

Validation

• cargo fmt
• git diff --check
• python scripts/check-coauthor-trailers.py --author-map .github/AUTHOR_MAP --range upstream/main..HEAD --check-authors
• cargo test -p codewhale-tui --bin codewhale-tui --locked command_discovery_tier_lists_use_canonical_registered_names -- --nocapture
• cargo test -p codewhale-tui --bin codewhale-tui --locked slash_completion_hints_hide_toolbox_commands_until_typed -- --nocapture
• cargo test -p codewhale-tui --bin codewhale-tui --locked slash_completion_hints -- --nocapture
• cargo test -p codewhale-tui --bin codewhale-tui --locked command_registry -- --nocapture
• cargo test -p codewhale-tui --bin codewhale-tui --locked command_info -- --nocapture
• cargo test -p codewhale-tui --bin codewhale-tui --locked
• cargo test --workspace --locked

Self-review

• Scope is limited to command discovery metadata and existing slash completion tests.
• No command dispatch, plugin registry, or alias behavior changes beyond hiding the canonical /plugin entry from root discovery.
• The new registry test fails if a future Advanced/Compatibility tier entry names an alias or a nonexistent command.