Maintain Codex image backend compatibility and harden CLI reliability

[jdmnk]

Changed

• Native Codex image generation/edit endpoints are now the default, following the Codex request format. The backend controls the actual image model.
• The original streamed backend remains available with --backend responses. Reasoning --model/--profile apply only to that backend and do not select the image generator.
• Remove the experimental --image-model flag and CODEX_IMAGEGEN_IMAGE_MODEL override: live requests also accepted a nonexistent model name without returning model identity, so model selection could not be verified.
• Explicit dimensions are validated against image-size constraints. Actual dimensions are reported; --size-policy warn|error controls mismatches without resizing.
• Unsupported transparency on the default path is rejected before contacting the backend.
• Refresh Pillow, the publishing toolchain and all locked dependencies; constrain Hatchling to the tested minor release and update CI actions/Python coverage.

Fixed

• Preserve raw JWT ID tokens during refresh; write credentials atomically with restricted POSIX permissions, coordinate imagegen refreshes and detect external credential changes.
• Validate and atomically save complete PNG/WebP outputs; protect existing files on failures and publish each successful path immediately.
• Apply EXIF orientation before edit-input compaction and reject corrupt inputs.
• Parse TOML model/profile settings correctly; explain file-only credential support.
• Normalize batch inputs, reject non-string prompts and escaped outputs, and honor continuation for per-job validation failures.
• Handle CRLF and split UTF-8 in SSE, terminal response results, truncated transport reads, HTTP 429/Retry-After and rate limits after authentication refresh.
• Validate positive finite timeouts and present expected filesystem failures as CLI errors.

Added

• Promote audit regressions to the normal suite and add native-backend, concurrency, persistence, output and retry coverage.
• Add opt-in live generation/edit compatibility tests that never refresh shared credentials.