docs: document merge-method rule and broken-link caveat in CLAUDE.md

[ecunado]

Adds guidance to CLAUDE.md to prevent the two issues we just hit:

1. Merge method per branch — feature → dev uses squash; dev → main must use a merge commit (never squash), so the long-lived branches keep shared history and don't produce phantom conflicts. Includes the recovery steps if a dev → main PR was accidentally squashed.

2. Broken-link caveat — the build uses onBrokenMarkdownLinks: 'throw', so intra-plugin links must be relative to the plugin (restobjects.md#anchor, not restapi/restobjects.md#anchor). Run yarn build locally before a prod deploy.

All instructions in English.