v0.4.0

v0.4.0 — read-only git surface, settings popover, line-number gutters

Major additions:
- Read-only git surface: new git-reader capability exposes branches, commits,
  diffs, and blob history via HTTP API, with corresponding SPA views.
- Settings popover consolidates global preferences (theme, wrap) and a
  two-row desktop toolbar reorganises navigation actions.
- Line-number gutters in the file viewer and diff hunks for easier reading.

Fixes:
- Copy link now works on non-secure HTTP origins (clipboard API fallback).

Also refreshes README (trimmed archive section, updated go install example)
and restructures the OpenSpec capability layout.