Fix plan delegate harness cleanup

[asim]

Summary:

• Let agent Stop unblock Run so plan/delegate test agents terminate cleanly.
• Start plan/delegate harness services non-blockingly and stop them explicitly after each run.

Testing:

• go build ./...
• go test ./... (fails here because the environment routes loopback gRPC through an envoy that forbids loopback targets, and atlascloud stream credentials return 400)
• golangci-lint run ./...
• go test -v -race -cover ./internal/harness/plan-delegate -count=1 -timeout=60s

Closes #4527
Closes #4536

[Copilot]

Pull request overview

This PR targets the internal/harness/plan-delegate test/harness hangs by tightening lifecycle management: services are started non-blockingly and explicitly stopped, and agents are updated so Stop() can unblock a goroutine running Run().

Changes:

• Replace go svc.Run() with svc.Start() + explicit defer svc.Stop() for plan/delegate harness services to avoid leaving service goroutines running between tests/runs.
• Add an agentImpl.stopCh so Stop() can signal Run() to return instead of parking forever.
• Apply the same service lifecycle changes to plan/delegate end-to-end tests to reduce cross-test leakage.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
internal/harness/plan-delegate/main.go	Start task/notify services non-blockingly and ensure they are stopped after each harness run.
internal/harness/plan-delegate/main_test.go	Mirror the harness lifecycle changes in tests to prevent leaked service goroutines.
agent/agent.go	Add a stop signal channel so Stop() can unblock Run() for cleaner harness/test shutdown.

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