* simplify CODEOWNERS
* dont .gitignore AGENTS.md
* AGENTS.md to document previous CODEOWNERS responsibilities
* update from https://developers.mattermost.com/contribute/more-info/server/schema-migration-guide/
* CREATE INDEX CONCURRENTLY now vetted
* rewrite and move to README.md
* dont limit to 80 chars
* rewrite webapp AGENTS.md and add to README.md
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Co-authored-by: Mattermost Build <build@mattermost.com>
