Fix polling automation examples to gate agent startup

[neubig]

• A human has tested these changes.

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Why

Polling automation examples should not encourage prompt-preset runs that start an agent conversation on every cron tick. Local polling scenarios should perform deterministic API checks first and only create or resume a conversation when there is actionable work.

Summary

• Update polling-related automation catalog prompts/examples to prefer webhooks where available and require custom polling scripts for local cron fallbacks.
• Clarify GitHub/Slack monitor docs and READMEs: no matching work means save state, fire callback, and exit without starting a conversation.
• Fix polling-script and custom-automation callback helpers to fall back to OPENHANDS_API_KEY when AUTOMATION_CALLBACK_API_KEY is not injected, with regression coverage.

Issue

Fixes #296

How to Test

• uv run python scripts/sync_extensions.py --check
• uv run pytest -q
• uv run python -m py_compile skills/github-repo-monitor/scripts/main.py skills/slack-channel-monitor/scripts/main.py
• JSON-load all files under automations/catalog/*.json

Video/Screenshots

N/A - documentation/template and script-helper changes only.

Notes

This PR was created by an AI agent (OpenHands) on behalf of the user.

sync_extensions.py --check reported the existing non-blocking coverage warning for ./plugins/issue-duplicate-checker not being in any marketplace.

This PR description update was created by an AI agent (OpenHands) on behalf of Graham Neubig.