Upstream candidates: LoggingUndefined trigger guards + on-demand integration docs URL

[Patch76]

Two additions that came up in practice today, both verified on HA 2026.3.3. Happy to turn either into a PR if useful.

1. LoggingUndefined behavior in event-trigger conditions

HA uses LoggingUndefined (not strict Jinja2 Undefined) for missing dict attributes. This has a non-obvious implication for automation conditions:

Access	Result
trigger.event when no event trigger fired	LoggingUndefined — no error
LoggingUndefined is defined	False — valid guard
'x' in LoggingUndefined.data.entity_id	UndefinedError
.split() on LoggingUndefined	UndefinedError

The error doesn't occur at attribute access — it occurs when operators like in or methods like .split() are called on the resulting LoggingUndefined object. is defined is therefore a safe guard, and trigger.platform == 'event' is the idiomatic HA-documented alternative (short-circuits before trigger.event is evaluated).

This would fit naturally in references/automation-patterns.md as a note under event-trigger conditions.

2. On-demand integration docs URL

https://raw.githubusercontent.com/home-assistant/home-assistant.io/refs/heads/current/source/_integrations/{domain}.markdown

Returns full HA integration documentation as plain text (HTTP 200, no auth required). Useful when an agent needs service call signatures or entity attributes for an unfamiliar integration — no caching or pre-loading needed, just substitute {domain}. Already referenced in the new domain-docs.md from PR #25, but might be worth a line in the main SKILL.md trigger table as a general-purpose lookup pattern.

Both are small additions — flagging here in case they're useful before investing in a PR.

[Patch76]

Turned this into a PR: #45 — covers the LoggingUndefined guard. The domain-docs URL (point 2) is already handled by PR #25 (references/domain-docs.md).