Update requirements spec for direct trigger mode

[liplus-lin-lay]

purpose

Update the requirements spec so it matches the merged direct trigger and notify-only fallback behavior.

premise

docs/0-requirements.md still describes only webhook persistence plus MCP polling.
The implementation now also supports serialized direct trigger execution, trigger status metadata, and a notification-only fallback path.

constraints

• Requirements must remain the source of truth for the merged behavior.
• Architecture, functional requirements, configuration, and file outputs must all stay consistent with the code and README.
• This issue is docs/spec only.

completion condition

• docs/0-requirements.md describes direct trigger execution, trigger metadata, and notify-only fallback.
• The updated spec matches the current merged implementation.