ci: add workflow_dispatch and retrigger scan

[danielcuthbert]

Adds manual trigger support and retriggers scan now API key secret is in place.

[github-actions]

OpenAnt Security Scan

🟢 No confirmed vulnerabilities

Metric	Value
Units scanned
Confirmed vulnerable
Bypassable controls
Inconclusive
LLM cost	$0.00

Stage 1 detection + Stage 2 attacker simulation via OpenAnt.
See the Security > Code scanning tab for annotated findings.

_{commit 8a0f681f67397504123a57afb5966111240610cf}