v0.13.1: Signed macOS Builds, a Postgres Explain That Finally Runs, and Pagination That Honors Your OFFSET

v0.13.1 is a correctness pass on v0.13.0: macOS builds are now code-signed and notarized, the Postgres Explain Plan that never worked now runs, paginated queries stop dropping your OFFSET, postgresql:// and mariadb:// connection strings are accepted, the MCP read-only gate stops misreading a parenthesized SELECT as a write, the grid no longer freezes on giant JSON cells, and the macOS Keychain stops prompting on every AI-tab open.