ci: harden CI/CD security

[jpr5]

Summary

• SHA-pin all GitHub Actions with version comments
• Add least-privilege permissions blocks to all workflows
• Fix shell injection (route attacker-controllable values through env)
• Add persist-credentials: false on read-only checkouts
• Add zizmor static analysis for workflow security
• Update Dependabot to daily for github-actions (auto-merge minor/patch)

Part of Phase 2 CI/CD supply chain hardening.

[jpr5]

Opened against wrong repo (fork). Recreating against CopilotKit/agentcore-cli.