Add direct trigger mode with notify-only fallback

[liplus-lin-lay]

purpose

Enable direct webhook-triggered Codex reactions while preserving a lightweight notification-only fallback for foreground polling.

premise

The MCP polling flow is lightweight but not immediate.
Direct codex exec on every webhook is possible, but resuming a Codex thread for each event is expensive and not suitable for notification-first workflows.

constraints

• Trigger execution must run one-by-one.
• Stored events must keep enough metadata to distinguish succeeded, failed, and skipped trigger handling.
• The bundled Codex wrapper must support both direct execution and resume mode.
• A workspace must be able to opt into notification-only fallback without editing the service command each time.
• Tests and operator docs must be updated in the same change.

completion condition

• main.py webhook accepts an optional trigger command and records trigger execution status.
• codex_reaction.py exists as a bundled wrapper for direct Codex execution.
• Notification-only fallback leaves events pending for foreground polling.
• README, .env.example, and tests cover the new behavior.