v0.1.0

Released to Maven Central as net.openan.workflow.sdk:*:0.1.0.

• Send SSE heartbeat comments on streaming endpoints (message:stream, tasks/{id}:subscribe) at a configurable
  interval (a2at.server.heartbeat-interval-seconds, default 15 seconds, 0 disables). Keeps intermediaries
  from idle-timing-out long-running SSE connections during agent execution; the heartbeat integrates with the
  emitter lifecycle and stops on stream completion, error, timeout, or client disconnect.
• Make collection fields on model types defensively copied and expose them through unmodifiable
  views (Workflow, WorkflowStep, WorkflowSearchResult, ExecutionResult, SendMessageResult,
  TaskResult, BusinessFailure, and the message records). Source- and binary-compatible; callers
  that mutate the collections returned by these getters now receive UnsupportedOperationException
  instead of silently corrupting engine-owned snapshots.
• Template the sample OMC agent credentials file: the tracked
  samples/src/main/resources/spn_agent_credentials.json is replaced by
  spn_agent_credentials.example.json (real file is now git-ignored; copy the example to run the
  demos). Missing credentials now fail with an actionable message instead of a bare NPE.