Add clipboard writeText and readText APIs

Editor assessment

The subject names the new APIs exactly: clipboard writeText and readText added to core. Type feat and scope core are accurate, the method names make the change verifiable, and no body is needed because the capability is fully described by its own names. No footers beyond the PR number. Hygiene is clean: imperative, short, no ticket junk. A tight, complete message for a well-bounded, user-facing API addition.