[IMP][P0] CI: validate all example templates on every PR

[mishrapravin114]

Type

Implementation — merge a PR that changes CI. Not investigation.

Goal

Every template with agent.py must pass ./engine validate on CI — catches broken agents before merge.

Files to change

.github/workflows/ci.yml ← UPDATE validate job

Implementation checklist

• Discover all paths: examples/templates/*/agent.py + examples/templates/supervisors/*/agent.py + supervised_agreement_analysis
• Run ./engine validate <path> for each (script loop or matrix)
• Job completes in <10 minutes on ubuntu-latest
• Existing lint + test jobs unchanged

Current gap

CI only validates agreement_analysis and log_monitor (2 of 15 templates).

Done when

• PR merged; CI green on main
• Intentionally break one template locally → validate job fails (manual check OK)

Out of scope

• Frontend E2E tests
• ./engine run smoke tests (optional P1 follow-up)

Related

Supersedes doc-heavy parts of #19