fix: dedupe CreateThread initiator participants

[casey-brooks]

Summary

• Fixes E2E mismatch: CreateThread rejects initiator in participants but initiator should be auto-included #35.
• Allows CreateThread requests to include the initiator in participant_ids or participants without failing validation.
• Dedupes the auto-included initiator so it is persisted exactly once and remains first in the participant list.
• Adds migration 0004_enforce_thread_delete_cascades.sql to enforce ON DELETE CASCADE on thread-owned rows (thread_participants, messages, message_recipients) so hard-deleting a thread cleans child rows instead of tripping FK constraints.
• Investigated agents E2E run 26379450519: the primary failure was CreateThread returning InvalidArgument for an initiator included in participant_ids; the cleanup log showed DeleteAgent returning FailedPrecondition because that failed flow left agent-related cleanup running after thread setup failed.

Validation

• buf generate buf.build/agynio/api --path agynio/api/threads/v1 --path agynio/api/notifications/v1 --path agynio/api/identity/v1 --path agynio/api/metering/v1 --path agynio/api/agents/v1 --path agynio/api/authorization/v1
• GOTOOLCHAIN=auto go test -json -count=1 ./...
• GOTOOLCHAIN=auto go vet ./...
• GOTOOLCHAIN=auto go build ./...
• Applied all SQL migrations against a local Postgres 16 container and verified deleting a thread cascades to thread participants, messages, and message recipients.

Test & Lint Summary

• Tests: 63 passed / 0 failed / 0 skipped.
• Lint: go vet ./... passed with no errors.
• Build: go build ./... passed.

[casey-brooks]

Test & Lint Summary

• buf generate buf.build/agynio/api --path agynio/api/threads/v1 --path agynio/api/notifications/v1 --path agynio/api/identity/v1 --path agynio/api/metering/v1 --path agynio/api/agents/v1 --path agynio/api/authorization/v1
• GOTOOLCHAIN=auto go test -json -count=1 ./...
  • Tests: 63 passed / 0 failed / 0 skipped.
• GOTOOLCHAIN=auto go vet ./...
  • Lint: passed with no errors.
• GOTOOLCHAIN=auto go build ./...
  • Build: passed.
• Applied all SQL migrations against a local Postgres 16 container and verified deleting a thread cascades to thread_participants, messages, and message_recipients.

[noa-lucent]

Review complete. The CreateThread change now dedupes the initiator while preserving the initiator-first ordering, and the added tests cover both participant_ids and participant identifiers. I did not find blocking issues.