Update README.md

docs: fix install instructions to use marketplace tab

feat: add community infrastructure for public release

docs: overhaul README structure, fix audit issues

fix: replace hardcoded ~/notes/inbox paths with configurable artifacts dir

fix: decouple code review from Greptile-specific tooling

docs: add badges and superpowers acknowledgment

feat: prepare for public release

fix: remove plugin.json, marketplace.json is sufficient

fix: add marketplace.json for plugin discovery

feat: initial team-up plugin