ci: replace webfactory/ssh-agent with inline ssh-agent setup

[jbachorik]

Summary

• webfactory/ssh-agent is a third-party action now blocked by org policy, causing startup_failure on the release-validated workflow before any job runs
• Replaces it with equivalent inline shell commands (ssh-agent, ssh-add, ssh-keyscan) in a run: step — same SSH_PUSH_SECRET key, same authentication behaviour, no third-party dependency

Test plan

• Trigger release-validated dry-run from main and verify it reaches the create-release job and authenticates SSH pushes successfully

🤖 Generated with Claude Code

[dd-octo-sts]

CI Test Results

Run: #26506567886 | Commit: 03d3307 | Duration: 12m 36s (longest job)

✅ All 32 test jobs passed

Status Overview

JDK	glibc-aarch64/debug	glibc-amd64/debug	musl-aarch64/debug	musl-amd64/debug
8	-	✅	-	-
8-ibm	-	✅	-	-
8-j9	✅	✅	-	-
8-librca	-	-	✅	✅
8-orcl	-	✅	-	-
11	-	✅	-	-
11-j9	✅	✅	-	-
11-librca	-	-	✅	✅
17	✅	✅	-	-
17-graal	✅	✅	-	-
17-j9	✅	✅	-	-
17-librca	-	-	✅	✅
21	✅	✅	-	-
21-graal	✅	✅	-	-
21-librca	-	-	✅	✅
25	✅	✅	-	-
25-graal	✅	✅	-	-
25-librca	-	-	✅	✅

Legend: ✅ passed | ❌ failed | ⚪ skipped | 🚫 cancelled

Summary: Total: 32 | Passed: 32 | Failed: 0

---

Updated: 2026-05-27 11:03:26 UTC