Moves developer, testing and release instructions out of the README,
minus the fixture-stripping advice, which was wrong.
Explains that a GitHub PR closes itself as merged once the mirror syncs,
because the merge preserves the contributor's SHAs -- so a PR closing
without a visible Merge click reads as success rather than rejection.
Documents the cog-api dependency: its CI clones this package at
USCOGDATA_REF, defaulting to main with no pin, so anything merged here
reaches the API's next build. Includes the commands to run its suite
against a branch first.
