List: sortable ordering (age / updated / size / priority)

[mayfieldiv]

Problem

The v1 PRD (#1, user story 10) calls for sortable ordering by size, age, last
updated, and priority. The Rust TUI has no user-facing sort: within a
Smart-status group PRs keep their pooled input order, and Grouping::None is
a flat list in that same order (src/app/grouping.rs — smart_status_rows
"preserving input order within each tier"; grouped_rows only sorts group keys
alphabetically). There is no key to reorder PRs by any column.

Approach

Add a sort dimension applied to PRs within each group (and globally under
Grouping::None), independent of the grouping/tier order:

• Sort keys: age (created), updated (last activity), size
  (additions+deletions, ideally code-only via the File Category breakdown
  when enrichment is present), and priority (Tier order).
• A keybinding to cycle the active sort key, and one to flip direction, with the
  active sort surfaced in the status bar / a chip (mirror the Substring
  Filter chip pattern in src/view.rs).
• Sorting composes with grouping: it orders members inside each group; tier/group
  ordering itself is unchanged. PRs still loading enrichment (no tier/size yet)
  sort last within their bucket rather than jumping around as data streams in.
• Persist the default sort key/direction in config (ui block,
  src/config.rs), matching how grouping defaults are handled.

Keep the ordering a pure function over the visible-index list so it is
table-test friendly, consistent with src/app/grouping.rs.

Open Questions

• Which keys? g is taken by the grouping cycle; s for sort-key cycle and S
  for direction is the obvious pairing, but confirm no conflict with existing
  bindings.
• Should size use total churn or code-only churn by default? Code-only matches
  the PRD's "real change size vs. noise" intent (stories 21-23).

Acceptance Criteria

• A keybinding cycles the sort key (age / updated / size / priority) and a
  modifier/second key flips direction.
• The active sort key + direction is visible in the UI.
• Sorting reorders PRs within each group and across the flat list, leaving
  group/tier ordering intact.
• PRs without the data a sort key needs sort deterministically (last) rather
  than reordering as enrichment arrives.
• Default sort persists via config.
• Pure ordering function with table-driven tests covering each key, both
  directions, missing-data, and ties.

References

• v1 PRD PRD: Legit — GitHub PR Triage TUI #1, user story 10 (and 21-23 for code-only size).
• src/app/grouping.rs, src/config.rs (ui), src/view.rs (filter chip),
  CONTEXT.md — Smart-status, Tier, File Category.