v0.7.3 - docs patch for upgrade auto-refresh

Pure docs patch — no code changes.

• skill.md now has an "Upgrading dbread (v0.7.2+)" subsection so when users ask Claude "how do I update dbread", Claude tells them uv tool upgrade dbread + restart Claude Code (no manual dbread install-skill --force mention).
• README line 60 removed the obsolete "Reinstall later with dbread install-skill --force after upgrading" advice; replaced with a one-liner about auto-refresh.

Same uv tool upgrade dbread flow as v0.7.2.