Desktop: file pane should remember pinned/open files across session restarts

Problem

When I close Claude Desktop (Code tab) and reopen the same session, the pane layout is restored but the content of the file pane is not — any .md / code file I had open is gone, and I have to re-locate the path in chat or the tree to reopen it.

For workflows where the same reference file is needed every day (a daily-review note, an architecture doc, a checklist), this is a constant small friction tax.

Proposed solution

Two complementary mechanisms:

1. Auto-restore last open file(s) — by default, the file pane should reopen whatever was open when the session was last closed/suspended. Same way the pane layout is restored.
2. Pin a file — right-click in the file-pane header → "Pin" — pinned files always reopen on session start, even after Discard/close. Stored per-session under ~/.claude/projects/<project>/<session>.json (or equivalent).

Scope

• Local sessions only (matches current file-pane availability — remote/SSH sessions excluded per docs).
• File pane only — the terminal pane (cwd) is already implicit via the session.

Why

• Pane-layout positioning already persists; file content is the missing half.
• The "pinned files" pattern matches VS Code, JetBrains IDEs, and Sublime — well-understood UX.
• Backend complexity is small — persist {filePath, scrollOffset} per file pane.

Environment

• Claude Desktop v1.2581.0 or later (per docs that mention layout-pane support).
• Affects all platforms; same request applies on macOS / Windows / Linux.

Related issues

• #43262 — workspace-level session save/restore (CLI; adjacent problem)
• #51746 — auto-open files and terminals for mentioned paths (different surface)
• #48911 — sidebar panel positioning (layout config)