Suppress Codex worker update prompts for unattended Maniple runs

[jacoblyles]

Summary

• set CI=1 when spawning Codex workers to suppress interactive update-install prompts that can block unattended remote runs
• preserve backend parity by applying the change in shared spawn flow before terminal backend launch (affects both tmux and iTerm)
• add tests for Codex env injection and env merging with worktree tracker vars

Why

Codex workers can pause for an interactive update prompt. When Maniple is used remotely/unattended, that prompt hangs the worker and prevents normal orchestration.

Backend parity: tmux + iTerm

• Backend parity: tmux + iterm
• Tests cover both backends (shared launch path in spawn_workers)

Test Results

• uv run pytest tests/test_cli_backends.py passes
• uv run pytest tests/test_spawn_workers_defaults.py (fails during collection due pre-existing circular import in this checkout: registry <-> utils.errors)

PR Checklist

• Backend parity: tmux + iterm
• Tests cover both backends (or exception + follow-up issue link)
• uv run pytest passes