Bump OPENCODE_VERSION ARG from 1.14.19 to 1.14.20 to track the new
upstream release on npm.
Clarify the tagging convention in AGENTS.md: the first build on a new
opencode version uses the bare 'v{opencode_version}' tag (no letter
suffix). Letter suffixes (a, b, c, ...) are reserved for container-
level rebuilds on the same opencode version (CVE fixes, doc changes,
entrypoint bugs). The previous wording implied a letter was always
required, which was never the actual behaviour.
joakimp