cli: show, list and dashboard screens (CLI views stage 2) #318

Stage 2 of #316, after stage 1. Plan tasks 9–15 in docs/plans/2026-10-01-cli-views.md. Two MRs.

2a, show screens:

• gitutil.DiffNumstat for the Files section.
• mr show: State, Checks, Gates, Review; description; Commits, Files, Discussion; legend Unblock / Review / Merge / Read chosen from the gates.
• issue show: Labels, Assignee, Milestone; description; Discussion; legend filtered by write access.
• repo show: Clone, Head with checks, Release, a Mirror line only on error; open MRs, open issues, recent commits.
• build show: Steps with a glyph each; build log in the legend.

2b, lists and dashboard:

• mr list, issue list, build list as one section each, ● leading rows that wait on the viewer.
• dashboard: Review requested, Assigned issues, Open merge requests, Failed builds, Recent activity (5), Pinned. Passing builds collapse to one line. Server/Queues leave the terminal screen (admin stats has them; --json keeps them).

Each command's piped output is pinned with pinPlain before it changes.