Add `gh devlake start` command

[ewega]

Summary

Add a gh devlake start command that brings up stopped/crashed DevLake services for an existing deployment. This fills a lifecycle gap — the CLI has deploy (initial setup) and cleanup (teardown) but no way to restart services after a reboot, crash, or stop.

Motivating scenario: After a machine reboot, gh devlake status shows ❌ for config-ui (container exited), but offers no remediation. Users must manually find the docker-compose directory and run raw Docker commands.

Proposed Solution

Local deployments (Docker Compose)

1. Auto-detect deployment from state file (.devlake-local.json) or fall back to docker-compose.yml in cwd
2. Check Docker availability
3. Run docker compose up -d (idempotent — starts stopped containers, recreates exited ones, no-op for running ones)
4. Wait for backend health (/ping) with shorter timeout than deploy (~60s vs 6min — databases/volumes already exist)
5. Print service URLs and health status

Azure deployments (Container Instances)

1. Read container names + resource group from .devlake-azure.json
2. Check Azure CLI login
3. Run az container start for each container
4. Wait for backend health
5. Print endpoints

Key design decisions

• Uses docker compose up -d (not docker compose start) because up -d handles both stopped AND crashed/exited containers
• Shorter health timeout since databases and volumes already initialized
• State file is NOT modified — Docker/Azure is the source of truth for runtime state

Flags

Flag	Default	Description
--service <name>	(all)	Start only a specific service (e.g., --service config-ui)
--no-wait	false	Skip health polling after start
--json	false	Machine-readable output

Scope of Changes

File	Change
cmd/start.go	New command implementation
cmd/deploy.go	Register start in operate command group
cmd/status.go	Add remediation hint when services are unhealthy (see #97)
docs/start.md	New documentation
docs/day-2.md	Add start to operations list
README.md	Add to command reference table

Acceptance Criteria

• gh devlake start brings up all services for a local deployment
• gh devlake start --service config-ui restarts only config-ui
• gh devlake start works for Azure deployments (reads state file)
• Health polling with appropriate timeout
• --json output supported
• Graceful error when no state file or docker-compose.yml found
• gh devlake status shows hint: "Run gh devlake start" when services are unhealthy

Dependencies

• Blocks: gh devlake stop (Fix temp directory leak in Azure fork deploy #97 — stop is the counterpart)

References

• cmd/deploy_local.go — startLocalContainers() function contains reusable patterns
• internal/docker/build.go — ComposeUp() already exists
• internal/devlake/discovery.go — health polling and endpoint inference