feat: scheduled primitives — extension model + standalone activity support

[chaptersix]

Summary

Extends the Schedule API to support multiple action primitives, with standalone activities as the first non-workflow primitive. Adds the four extension seams described in the Scheduled Primitives Design:

1. A new ScheduleAction.start_activity oneof variant referencing NewActivityExecutionInfo.
2. Generic extension types — ScheduleActionStatus, ScheduleActionKind, ScheduledExecutionReference, ScheduleActivityExecution — that replace per-primitive field growth on ScheduleActionResult, ScheduleInfo, and ScheduleListInfo.
3. New generic ScheduleActionResult fields (start_execution_result, start_execution_status, start_execution_link) used by all non-workflow action types.
4. A typed Link.Schedule variant so the scheduler can be a typed link target rather than overloading Link.WorkflowEvent.

Existing workflow-specific fields are frozen for backward compatibility:

• ScheduleAction.start_workflow (field 1)
• ScheduleActionResult.start_workflow_result (11) / start_workflow_status (12)
• ScheduleInfo.running_workflows (9)
• ScheduleListInfo.workflow_type (2)

Commits

• feat: add Link.Schedule variant for bi-directional schedule linking — adds the typed schedule link variant, modeled after Link.Activity (also a CHASM component without an event history).
• feat: add schedule extension types for standalone activity actions — adds ScheduleActionStatus, ScheduleActionKind, ScheduledExecutionReference, ScheduleActivityExecution, the start_activity oneof variant, and generic fields on ScheduleActionResult / ScheduleInfo / ScheduleListInfo.
• feat: add NewActivityExecutionInfo message for schedule actions — shared config message for starting a standalone activity in different contexts (mirrors NewWorkflowExecutionInfo). Carries activity-specific config; request-level fields stay on StartActivityExecutionRequest.

Verification

• buf build — passes
• buf lint — passes
• buf breaking --against upstream/master — passes (no breaking changes)

Test plan

• Regenerate api-go bindings on top of this branch
• Wire start_activity dispatch through the scheduler (downstream PR)
• Manual e2e: create schedule with start_activity, observe running_executions and recent_actions[].start_execution_result

Stacked

PR 2 (alex/sch-primitives-nexus) stacks on this branch and adds start_nexus_operation as the second primitive.