docs: lead with /plugin install path in advisor README and setup.md

[jkzietz]

Addresses StartupEngBlend-3023. Both advisor/README.md and advisor/plugins/aws-startup-advisor/setup.md previously documented only the npx skills add install path. This change leads with the Claude Code plugin-install path so that installs through the documented flow register in the Claude Code plugin metric.

Changes

advisor/README.md

Added a "Claude Code (recommended)" section above the existing npx instructions:

/plugin install aws-startup-advisor@claude-plugins-official

The existing npx skills add content is preserved under a new "All other agents" subheading.

advisor/plugins/aws-startup-advisor/setup.md

Restructured the agent-facing install flow:

• Step 1: Detect environment (Claude Code vs. other)
• Step 2A: Claude Code → /plugin install aws-startup-advisor@claude-plugins-official (preferred)
• Step 2B: All others → Node.js check + npx skills add (unchanged)
• Step 3: Confirm success (unchanged)

What's preserved

• The npx skills add path is fully documented for non-Claude agents (Kiro, Cursor, Codex, Copilot)
• All per-agent examples, global install, and single-skill install instructions are unchanged

Verification

• markdownlint-cli2: 0 errors
• Documentation-only change, no runtime impact