docs(readme): Trim v2.6.0 Recent Updates to user-facing scope

Per references/readme-recent-updates.md ('Recent Updates is for
features, not bugs'), removed three internal-tier bullets from
the v2.6.0 entry:

- term.ps1 PowerShell port — internal infrastructure
- DESIGN.md → TERMINAL-DESIGN.md rename — housekeeping
- diagnostic-script exit-code normalisation — bug-tier fix

The windows-ops skill itself is the user-facing announcement;
also trimmed the trailing 'Bug exposed and fixed during the
visual refactor: ...' sentence from the windows-ops bullet for
the same reason (implementation detail, not a feature). The
commit history retains the full provenance.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>