feat: add GTM Signal Intelligence Agent template (#5817)

[n2400813g]

Summary

This PR implements the GTM Signal Intelligence Agent template as described in issue #5817.

Changes

• Created the gtm_signal_agent template under examples/templates/.
• Implemented core integrations (Exa/news, Apollo, HubSpot) with initial mock interfaces.
• Designed a 9-node event loop graph incorporating human-in-the-loop (HITL) points for intake, outreach approval, and weekly digests.
• Fixed a framework bug in core/framework/graph/event_loop_node.py causing an UnboundLocalError when LLM calls fail early in a turn.

Testing

• Tested in TUI mode.
• Validated with mock integration data.

[github-actions]

PR Requirements Warning

This PR does not meet the contribution requirements.
If the issue is not fixed within ~24 hours, it may be automatically closed.

PR Author: @n2400813g
Found issues: #5817 (assignees: none)
Problem: The PR author must be assigned to the linked issue.

To fix:

1. Assign yourself (@n2400813g) to one of the linked issues
2. Re-open this PR

Exception: To bypass this requirement, you can:

• Add the micro-fix label or include micro-fix in your PR title for trivial fixes
• Add the documentation label or include doc/docs in your PR title for documentation changes

Micro-fix requirements (must meet ALL):

Qualifies	Disqualifies
< 20 lines changed	Any functional bug fix
Typos & Documentation & Linting	Refactoring for "clean code"
No logic/API/DB changes	New features (even tiny ones)

Why is this required? See #472 for details.