Pinned Items — pin Projects, Views & Milestones to the sidebar [GA]

### Value Prop
Pinned Items lets you save up to 25 projects, views, and milestones directly in the repository sidebar, so your most-used destinations are always one click away. Whether you're tracking a milestone, jumping back to a saved issue query, or checking in on a project board, you get there without navigating through menus every time. Your sidebar becomes a persistent launchpad tailored to how you actually work.

### Expected Outcome
Saved views are powerful, but only if people come back to them — and data from the preview showed that most users who created a view never returned to it. Pinned Items closes that gap by keeping the things you care about visible and reachable in the sidebar at all times. The goal is to make saved views, projects, and milestones a natural part of the daily workflow, not a feature people set up once and forget.

Start with issue #1292 and review the existing repository sidebar, saved views, projects, and milestones workflows. Define how users pin and unpin these destinations, how the 25-item limit behaves, and what the completed sidebar experience should support; the issue does not identify implementation files or tests.

索引モデルが issue の本文から書いたものです。