fix(codex): usable-today caveats for max reasoning level and bare gpt-5.6 alias

Live probes against the OpenAI API (2026-07-09):
- -c model_reasoning_effort=max works only on codex CLI >= 0.144 AND an explicit
  GPT-5.6 model (-m gpt-5.6-sol). Installed 0.136 rejects the config value
  ('unknown variant max'); non-5.6 default models return API 400.
- Bare '-m gpt-5.6' is rejected for Codex with ChatGPT sign-in even though the
  API-level alias to gpt-5.6-sol is real (OpenAI API changelog 2026-07-09).
- An 'ultra' tier exists above max on some surfaces (ChatGPT model picker,
  codex-rs ReasoningEffort enum).

Template + regenerated SKILL.md in one commit (check-freshness bisect invariant).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

codex/SKILL.md

@@ -1537,8 +1537,9 @@ The reason must engage with a specific Codex insight and compare against an alte
 
 **Model:** No model is hardcoded — codex uses whatever its current default is (the frontier
 agentic coding model). This means as OpenAI ships newer models, /codex automatically
-uses them. As of July 2026 that frontier is the GPT-5.6 family (ladder: `sol` > `terra` > `luna`,
-where bare `gpt-5.6` aliases to `sol`). If the user wants a specific model, pass `-m` through to codex.
+uses them. As of July 2026 that frontier is the GPT-5.6 family (ladder: `sol` > `terra` > `luna`;
+the bare `gpt-5.6` API alias routes to `sol`, but Codex with ChatGPT sign-in rejects the bare
+id — pass the full `gpt-5.6-sol`). If the user wants a specific model, pass `-m` through to codex.
 
 **Reasoning effort (per-mode defaults):**
 - **Review (2A):** `high` — bounded diff input, needs thoroughness but not max tokens
@@ -1548,8 +1549,11 @@ where bare `gpt-5.6` aliases to `sol`). If the user wants a specific model, pass
 `xhigh` uses ~23x more tokens than `high` and causes 50+ minute hangs on large context
 tasks (OpenAI issues #8545, #8402, #6931). Users can override with `--xhigh` flag
 (e.g., `/codex review --xhigh`) when they want maximum reasoning and are willing to wait.
-The GPT-5.6 family adds a `max` level above `xhigh` — even more token-hungry, so gstack
-wires no flag for it; pass `-c model_reasoning_effort=max` manually if you truly need it.
+The GPT-5.6 family adds a `max` level above `xhigh` (and an `ultra` above that on some
+surfaces) — OpenAI positions both as trading latency and cost for depth, so gstack wires
+no flag for them. If you truly need it: codex CLI >= 0.144 with an explicit GPT-5.6 model
+(e.g. `-m gpt-5.6-sol -c model_reasoning_effort=max`) — older CLIs reject the value and
+non-5.6 models return a 400.
 
 **Web search:** All codex commands use `--enable web_search_cached` so Codex can look up
 docs and APIs during review. This is OpenAI's cached index — fast, no extra cost.

codex/SKILL.md.tmpl

@@ -606,8 +606,9 @@ The reason must engage with a specific Codex insight and compare against an alte
 
 **Model:** No model is hardcoded — codex uses whatever its current default is (the frontier
 agentic coding model). This means as OpenAI ships newer models, /codex automatically
-uses them. As of July 2026 that frontier is the GPT-5.6 family (ladder: `sol` > `terra` > `luna`,
-where bare `gpt-5.6` aliases to `sol`). If the user wants a specific model, pass `-m` through to codex.
+uses them. As of July 2026 that frontier is the GPT-5.6 family (ladder: `sol` > `terra` > `luna`;
+the bare `gpt-5.6` API alias routes to `sol`, but Codex with ChatGPT sign-in rejects the bare
+id — pass the full `gpt-5.6-sol`). If the user wants a specific model, pass `-m` through to codex.
 
 **Reasoning effort (per-mode defaults):**
 - **Review (2A):** `high` — bounded diff input, needs thoroughness but not max tokens
@@ -617,8 +618,11 @@ where bare `gpt-5.6` aliases to `sol`). If the user wants a specific model, pass
 `xhigh` uses ~23x more tokens than `high` and causes 50+ minute hangs on large context
 tasks (OpenAI issues #8545, #8402, #6931). Users can override with `--xhigh` flag
 (e.g., `/codex review --xhigh`) when they want maximum reasoning and are willing to wait.
-The GPT-5.6 family adds a `max` level above `xhigh` — even more token-hungry, so gstack
-wires no flag for it; pass `-c model_reasoning_effort=max` manually if you truly need it.
+The GPT-5.6 family adds a `max` level above `xhigh` (and an `ultra` above that on some
+surfaces) — OpenAI positions both as trading latency and cost for depth, so gstack wires
+no flag for them. If you truly need it: codex CLI >= 0.144 with an explicit GPT-5.6 model
+(e.g. `-m gpt-5.6-sol -c model_reasoning_effort=max`) — older CLIs reject the value and
+non-5.6 models return a 400.
 
 **Web search:** All codex commands use `--enable web_search_cached` so Codex can look up
 docs and APIs during review. This is OpenAI's cached index — fast, no extra cost.