feat(stovepipe): add start controller for the change ingress flow

[mnoah1]

Setting up start controller. Per our discussions, the gateway will be responsible for polling and finding new commits, then enqueuing them in order for the orchestrator to pick up.

Start will kick off the rest of the workflow for a commit.

Following SubmitQueue pattern of having Gateway set the partition key when creating the event, instead of deriving it internally within the orchestrator.

Why?

Initial step for commit validation workflow

What?

Setting up "start" step of this workflow: https://github.com/uber/submitqueue/blob/main/doc/rfc/stovepipe/workflow.md

Test Plan

• Wire into example app
• Manually inject an event via sql, confirm that that the event triggers, and inserts a validate message

rchestrator-service-1  | 2026-06-16T14:31:29.811Z	DEBUG	queue_mysql.message_store	mysql/message_store.go:217	fetched messages	{"topic": "start", "partition_key": "git.example.com/uber/monorepo", "count": 1}
orchestrator-service-1  | 2026-06-16T14:31:29.815Z	DEBUG	consumer/consumer.go:351	processing delivery	{"controller": "start", "topic_key": "start", "message_id": "test-msg-1", "partition_key": "git.example.com/uber/monorepo", "attempt": 1}
orchestrator-service-1  | 2026-06-16T14:31:29.815Z	INFO	start_controller	start/start.go:85	received change event	{"uri": "git://git.example.com/uber/monorepo/refs%2Fheads%2Fmain/abcdef0123456789abcdef0123456789abcdef01", "attempt": 1, "partition_key": "git.example.com/uber/monorepo"}
orchestrator-service-1  | 2026-06-16T14:31:29.816Z	DEBUG	queue_mysql.message_store	mysql/message_store.go:72	inserting messages	{"topic": "validate", "count": 1, "visible_after": 0}
orchestrator-service-1  | 2026-06-16T14:31:29.821Z	DEBUG	queue_mysql.message_store	mysql/message_store.go:125	inserted messages	{"topic": "validate", "count": 1}
orchestrator-service-1  | 2026-06-16T14:31:29.821Z	DEBUG	queue_mysql.publisher	mysql/publisher.go:65	published message	{"topic": "validate", "message_id": "git://git.example.com/uber/monorepo/refs%2Fheads%2Fmain/abcdef0123456789abcdef0123456789abcdef01"}
orchestrator-service-1  | 2026-06-16T14:31:29.821Z	INFO	start_controller	start/start.go:96	published commit to validate	{"uri": "git://git.example.com/uber/monorepo/refs%2Fheads%2Fmain/abcdef0123456789abcdef0123456789abcdef01", "topic_key": "validate"}
orchestrator-service-1  | 2026-06-16T14:31:29.825Z	DEBUG	consumer/consumer.go:479	message processed successfully	{"controller": "start", "topic_key": "start", "message_id": "test-msg-1", "partition_key": "git.example.com/uber/monorepo", "attempt": 1, "elapsed_ms": 5}
orchestrator-service-1  | 2026-06-16T14:31:29.900Z	DEBUG	queue_mysql.partition_lease_store	mysql/partition_lease_store.go:191	retrieved leased partitions	{"topic": "start", "count": 1}
orchestrator-service-1  | 2026-06-16T14:31:29.900Z	DEBUG	queue_mysql.subscriber_heartbeat_store	mysql/subscriber_heartbeat_store.go:95	found active subscribers	{"topic": "start", "count": 1, "subscribers": ["orchestrator-dev"]}
orchestrator-service-1  | 2026-06-16T14:31:29.901Z	DEBUG	queue_mysql.partition_lease_store	mysql/partition_lease_store.go:232	discovered partitions	{"topic": "start", "count": 1}
orchestrator-service-1  | 2026-06-16T14:31:29.901Z	DEBUG	queue_mysql.partition_lease_store	mysql/partition_lease_store.go:83	acquired lease	{"topic": "start", "partition_key": "git.example.com/uber/monorepo"}