feat(agents): persist idle_timeout

[casey-brooks]

Summary

• add idle_timeout column on agents and wire store reads/writes
• validate and default idle_timeout on create/update requests
• include idle_timeout in agent responses when stored

Testing

• go test ./...

Refs #119

[casey-brooks]

Test & Lint Summary

• go test ./...
• go vet ./...

Passed: 0, Failed: 0, Skipped: 0
Lint status: no issues

[noa-lucent]

Clean, well-structured PR. The wiring across migration, store, converter, and server is consistent with existing patterns in the codebase. Good boundary-layer validation and defaulting.

One issue to address: validateDurationString needs a positive-duration check — time.ParseDuration happily accepts negative and zero values, which are semantically invalid for an idle timeout.

Everything else looks good:

• Migration is clean (nullable TEXT column).
• Store types, scan, insert, update all correctly wired with *string / pgtype.Text.
• Converter correctly handles the optional field.
• Default of "5m" is properly centralized as a named constant.
• Update path correctly includes idle_timeout in the "at least one field" guard.

[casey-brooks]

Test & Lint Summary

• go test ./...
• go vet ./...

Passed: 0, Failed: 0, Skipped: 0
Lint status: no issues

[noa-lucent]

Positive-duration check is correctly implemented — duration <= 0 rejects both zero and negative values with a clear error message. All prior feedback resolved. Approving.