ci: add Gitleaks secret scan

[adamkoy]

Summary

• Adds the public gitleaks/gitleaks-action v3.0.0 on pull requests, pushes to main, and a monthly schedule.
• Same workflow as foundry-template#9. The org secret GITLEAKS_LICENSE is already set.
• PR comments and SARIF upload are off so a finding is not published on a public pull request. The job still fails if a leak is found.

Test plan

• Secret Scan (Gitleaks) runs on this PR

[CLAassistant]

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[github-actions]

Changes to gas cost

Generated at commit: 3a49b8714ca86e3901895d646bb8ac29d4fc2816, compared to commit: 81ec16e7199d01a988d573a04e7b3b2991aa7c31

🧾 Summary (20% most significant diffs)

Contract	Method	Avg (+/-)	%

---

Full diff report 👇

Contract	Deployment Cost (+/-)	Method	Min (+/-)	%	Avg (+/-)	%	Median (+/-)	%	Max (+/-)	%	# Calls (+/-)
ERC20ExtendedHandler	766,606 (0)	approve burn mint transfer transferFrom	31,355 (0) 41,828 (0) 381 (0) 477 (0) 488 (0)	0.00% 0.00% 0.00% 0.00% 0.00%	45,754 (-4) 46,293 (-10) 50,997 (+16) 57,621 (-29) 54,134 (-24)	-0.01% -0.02% +0.03% -0.05% -0.04%	51,303 (0) 44,691 (0) 62,063 (0) 60,896 (0) 60,925 (0)	0.00% 0.00% 0.00% 0.00% 0.00%	51,879 (0) 53,627 (0) 96,839 (0) 131,469 (0) 133,779 (0)	0.00% 0.00% 0.00% 0.00% 0.00%	12,854 (0) 12,951 (0) 12,964 (0) 12,670 (0) 12,812 (0)
ERC20ExtendedHarness	1,677,854 (-2,052)	approve burn mint transfer transferFrom	26,013 (0) 24,108 (0) 28,461 (0) 24,097 (0) 24,582 (0)	0.00% 0.00% 0.00% 0.00% 0.00%	41,137 (-7) 29,707 (-6) 50,625 (+13) 31,606 (-12) 35,402 (-9)	-0.02% -0.02% +0.03% -0.04% -0.03%	45,973 (0) 28,711 (0) 51,269 (0) 28,871 (0) 31,873 (0)	0.00% 0.00% 0.00% 0.00% 0.00%	46,537 (0) 34,695 (0) 68,873 (0) 51,955 (0) 57,769 (0)	0.00% 0.00% 0.00% 0.00% 0.00%	14,663 (-9) 13,466 (0) 15,342 (0) 11,828 (0) 11,133 (0)

[github-actions]

LCOV of commit 7e9e88f during Forge Coverage #191

Summary coverage rate:
  lines......: 95.4% (476 of 499 lines)
  functions..: 95.6% (153 of 160 functions)
  branches...: no data found

Files changed coverage rate: n/a