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