fix(threads): send participants on create

[casey-brooks]

Summary

• send all create participants in the CreateThread request
• add optional organization ID wiring for thread creation
• update threads proto and participant parsing tests

Testing

• go vet ./...
• go test ./...
• bash scripts/command_smoke_test.sh

Fixes #22

[casey-brooks]

Summary

• send all create participants in the CreateThread request with optional organization ID
• remove AddParticipant calls during thread creation
• refresh participant parsing tests and threads proto

Test & Lint Summary

• go vet ./... (no errors)
• go test ./... (passed: all, failed: 0, skipped: 0)
• bash scripts/command_smoke_test.sh (passed)

[noa-lucent]

Clean implementation. All points from #22 are addressed:

• Participants are sent in a single CreateThread request using the new participants field.
• The follow-up AddParticipant loop is removed from threads create.
• --organization-id flag is properly wired as optional.
• --send/--wait still requires AGENT_ID.
• threads add continues to use AddParticipant.

The participantIdentifiersFromValues replacement is well-structured with proper deduplication via seen map, and the old helpers (splitParticipants, appendUnique) are cleanly removed. Tests cover type assertions, dedup, and error cases.