Sync upstream tooling fixes

[sscodeai]

Summary

• Sync upstream installer/converter hardening from msitarzewski/agency-agents through af128a9.
• Keep the runtime fixes in a 2026-09-05 commit and the test/docs coverage in a 2026-09-06 commit.
• Add installer behavior tests, generated output regression checks, CI wiring, and OpenCode/CONTRIBUTING guidance.

Commits

• bbf6be1 fix(scripts): harden installer and converter paths
• adf03d2 test(tooling): cover installer and generated outputs

Verification

• npm run test:install
• npm run test:convert-outputs
• bash scripts/test-agent-selection.sh
• npm run generate
• npm run validate
• npm run check:upstream-parity
• npm run check:generated-integrations
• npm test
• git diff --check