fix(codex): drop SessionStart systemMessage that renders as a yellow warning (#605)

[spde]

Problem

On Codex, every SessionStart shows ponytail's mode as a yellow warning: line that reads like an error (#605):

• SessionStart hook (completed)
  warning: PONYTAIL:FULL
  hook context: PONYTAIL MODE ACTIVE — level: full

This is distinct from #331 / openai/codex#16933 (Codex rendering additionalContext) — this line is ponytail's own systemMessage.

Fix

Drop systemMessage in the isCodex branch of hooks/ponytail-runtime.js. Codex maps a hook systemMessage to a Warning entry, and any Warning entry also de-greens the completed-hook bullet (codex tui/src/history_cell/hook_cell.rs). Removing it:

• drops the warning: line,
• greens the completed-hook bullet,
• keeps every state visible via the hook context: line — level: <mode> when on, PONYTAIL MODE OFF when off (that path passes context too), since Codex renders additionalContext.

So no loss of visibility, only the alarming styling goes. The isCodex branch is now identical to isQoder — glad to dedup them if you'd prefer.

Tests

npm test — updated the codex assertions in tests/hooks.test.js to assert no systemMessage and confirm the mode/off state still surfaces via additionalContext. (The pre-existing tests/correctness.test.js csv failure is unrelated — it fails on clean main here too.)

Fixes #605

[AlexAgo83]

Confirming this on Codex CLI 0.147.0 / macOS with Ponytail 4.9.0.

Worth flagging that it isn't warning-only on this Codex version: the
systemMessage surfaces as actual hook failures, so every session opens with

SessionStart hook (failed)
  error: hook exited with code 1

UserPromptSubmit hook (failed)
  error: hook exited with code 1

Applying this patch — dropping the Codex systemMessage while keeping
hookSpecificOutput.additionalContext — clears both events, and Ponytail
still reports its mode through the hook context. Details and the before/after
output are in #605.