Use numeric entry labels and simpler locations in xmd repl #929

Story

As a person navigating xmd repl, I want entries identified by their number and copied locations to omit redundant defaults, so entry names and links are easier to read.

In an entry list or History, entry 19 is labeled 19; a sentence can say Entry 19. Its location uses /sessions/19 instead of /sessions/entry-19. A location showing the default Sidekick pane need not add right=sidekick.

Current gap

The entry catalog displays an ordinal followed by an internal-looking name such as entry-19, repeating the number without identifying the program. Locations also retain explicitly specified default pane choices.

The inspected location is otherwise coherent: Sessions and the selected entry are independent, and the long draft= value preserves unsent code. Its size does not establish duplicated navigation.

Contract

• Use numeric entry labels where the entry context is clear, and readable Entry N wording where context is needed.
• Use numeric public entry selectors in copied locations, including the selected entry alongside either surface.
• Omit redundant default pane selection from newly generated locations. Preserve explicit nondefault choices.
• Preserve the exact unsent draft and other meaningful selected state. Shortening a link must not discard editor content.
• Existing-link compatibility is explicitly unnecessary for this work. Do not add old-spelling aliases merely to preserve previous URLs.
• Keep saved sessions, source, outcomes, and submission-to-entry associations intact. Public numeric selectors can map to existing records; a label or URL change does not require rewriting execution history.

Reconcile the location grammar and its specification with the new spelling, including nested scopes and checkpoint references. This is a presentation/location change, not a new entry identity or history migration.

Acceptance

• Entry numbering appears once in an entry-selection row; internal entry-N names are not displayed as redundant titles.
• A copied numeric location restores the exact selected entry, surface, meaningful pane choice, checkpoint, and unsent draft.
• Default Sidekick locations omit the redundant pane query; a nondefault Bindings choice remains explicit.
• Distinct numeric selectors resolve to distinct entries. Invalid selectors refuse instead of selecting a different entry.
• Opening a copied location neither submits its draft nor repeats an earlier entry.
• Existing saved submission records still identify the work that actually ran.

Use the REPL route, entry-navigation, and cold-reopening tests with the new public spelling. Remove compatibility-only expectations for old links while preserving the reconstruction assertions.

Relationships and scope

Follow-up to #883 and PR #921, related to Quest #827. Coordinate labels with the separate History-navigation story, without combining their implementation scopes.

The local Architect assessment and explicit compatibility waiver are recorded in .reviewer/issue-883/pr921-ui-feedback-sessions-history-sidekick.md. The old catalog's names already exist at PR base 79d404d0.

This is separate follow-up work. A draft-free share-link mode, session deletion, provider identity changes, and rewriting saved execution records are outside scope.