fix: start agents synchronously, and add examples setting data on start

[mattjohnsonpint]

When starting an agent, it's a common need to set initial data. The easiest way to do this is by passing a message immediately after starting the agent. However, previously, there could be a race condition wherein the actor was not yet ready to receive the message because it was being started asynchronously.

This changes the normal agent startup to be synchronous, so immediately after starting an agent, the agent is guaranteed to be running.

Added some tests to prove it works.

Resuming agents on startup or reload will still be async.

[linear]

HYP-3422 Need ability to send initial data when starting an agent

[Copilot]

Pull Request Overview

This PR fixes a race condition by changing agent startup from asynchronous to synchronous and adds examples for starting agents with initial data. Key changes include:

• Adding new functions (StartCounterAgentWithData) in Go and AssemblyScript examples that set initial count data.
• Updating the agent startup flow in the runtime to use synchronous spawning.
• Adjusting the changelog to document these updates.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
sdk/go/examples/agents/main.go	Adds a function to start a counter agent with initial data.
sdk/assemblyscript/examples/agents/assembly/index.ts	Provides an AssemblyScript example for starting an agent with initial data.
runtime/actors/agents.go	Refactors agent startup to be synchronous using spawnActorForAgent.
runtime/actors/actorsystem.go	Adjusts actor restart and spawning in goroutines to support synchronous startup.
CHANGELOG.md	Updates changelog with the new sync startup feature.