Previous phrasing treated the letter suffix as a plain alphabetical
sequence, which led to confusion about whether the first rebuild
should be 'a' or 'b'. Spell out the intent: the suffix is the build
ordinal, and the letter 'a' is reserved to mean '1st build' — which
always uses the bare tag (no letter). So letters start at 'b' for
the 2nd build, 'c' for the 3rd, and so on.
Examples for opencode version 1.14.20:
1st build: v1.14.20
2nd build: v1.14.20b
3rd build: v1.14.20c
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.