feat: reuse version-floor + structured-error-classification when Codex/Antigravity adapters land

[Harikeshav-R]

Problem / motivation

PR #14 finished the Claude Code adapter with two riders: (a) structured error
classification from stream-json's error category, and (b) a CLI version floor.
When the Codex (codex exec) and Antigravity (agy) adapters are built
(deferred to a later phase — PROJECT.md §15, Appendix A.2/A.3), we should
deliberately reuse or adapt these rather than reinvent them. This issue captures
how each rider maps, so nothing is lost between now and then.

Rider (b) — CLI version floor: reuse as-is

parse_cli_version, CliAdapter._minimum_version(), and check_availability()
live in the base CliAdapter (src/atlas/ai/cli/base.py). A new adapter gets
them for free and only needs:

• override _minimum_version() → its own floor tuple;
• override _version_argv() iff codex --version / agy --version print a
  format the leading-semver regex doesn't already handle (it tolerates vX.Y.Z
  and trailing text, so it likely just works).

That's a 1–3 line override per adapter; atlas doctor already surfaces the reason.

Rider (a) — structured error classification: same idea, per-adapter mechanics

The principle carries over (classify from a structured signal; keep the stderr
heuristic as a fallback), but the signal and plumbing differ — neither is a copy
of Claude's stream-json rewrite:

	Claude Code (done)	Antigravity (agy)	Codex (codex exec)
Structured happy path	stream-json terminal result event	--output-format json envelope (structured_output)	--output-schema file / final stdout msg
Error signal	stream-json error category (authentication_failed, rate_limit, …)	status field in the plain JSON envelope (SUCCESS/ERROR/CANCELED/…)	--json JSONL error event
Needs a stream-json switch for errors?	Yes (was required)	No — status is already in the plain envelope	No — parse the --json JSONL stream

Notes:

• Antigravity is simpler than Claude here: key off status != "SUCCESS" in the
  plain envelope. But its statuses are coarse (not fine-grained auth-vs-rate-limit),
  so auth detection may still lean on the "authentication required" text.
• Codex is the most different: file-based structured output (-o out.json) and
  a JSONL error event.
• Event names/shapes differ across all three, so field extraction stays per-adapter.
  A shared "scan NDJSON events for a structured field" helper could be lifted into
  the base once ≥2 adapters exist.

Proposed solution

When implementing each adapter, override _minimum_version() (+ _version_argv()
if needed) for the floor, and implement _classify_error/_parse_response against
that CLI's own error signal (status for agy, JSONL error event for codex),
keeping the stderr-substring fallback. Consider extracting a shared NDJSON-scan
helper at that point.

Alternatives considered

Generalizing the classification now (before either adapter exists) — rejected: the
signals differ enough that a premature abstraction would be guesswork; better to
build the second adapter, then lift the commonality.

References

• PROJECT.md Appendix A.4 (note added in PR feat(ai): finish the AI provider layer (stream-json errors + version floor) #14), A.2 (Codex), A.3 (Antigravity)
• src/atlas/ai/cli/base.py (parse_cli_version, _minimum_version, check_availability)
• src/atlas/ai/cli/claude_code.py (reference implementation of both riders)