formula: imply `link_overwrite` for related formulae

Treat Homebrew-owned conflicts from related formula families as implied
`link_overwrite` paths when linking versioned or -full formulae.

This restores the version-switching behavior that regressed for cases
like installing `node@22` after uninstalling `node` left `npm` files
behind in the prefix.

While we're here, add some `AGENTS.md` changes based on feedback.

References Homebrew/homebrew-core#271139