docs: make installed commands the primary path

[aryamthecodebreaker]

Summary

• lead the README and website with a proper global install and the short ixmap plan ... command
• use the installed ixmap binary for CLI docs and MCP configuration
• keep npx as an explicitly resolution-sensitive one-off trial
• document the boundary proved after v0.8.4: an older project-local Doctor can win before new detector code starts

Verification

• web typecheck passed
• web lint passed
• Next.js production build passed with all static routes

Closes #437