docs(skill): document prerequisites and link setup guide

[lyingbug]

Problem

Users who copy SKILL.md on its own can reach bsk session start without the CLI or a connected browser extension. The skill does not point them to the existing setup guide.

Changes

• Add a short prerequisites section explaining that installing the skill does not install the CLI or extension.
• Point to the official installation guide when setup is needed, with a PATH check before reinstalling and bsk --version / bsk doctor verification.
• Keep the repository skill and the copy bundled with the CLI identical.

Verification

• Cross-checked the setup guidance against AGENT_INSTALL.md and the installer scripts.
• Skill metadata validation passed for both changed files.
• Confirmed both skill copies are byte-for-byte identical with cmp.
• git diff --check and git diff --cached --check passed.
• Documentation-only change; installers, live browser setup, and runtime test suites were not run.