Add taskModelSource to SubagentStartedData

[bhavyaus]

Summary

Adds the Node event type for the runtime's new optional taskModelSource field on subagent.started (github/copilot-agent-runtime#18727). VS Code Agent Host reads it to report where a task-tool sub-agent's model input came from.

Value	What it tracks
task_argument	The parent agent passed task.model
subagent_configuration	No task.model; the per-sub-agent settings entry supplied a concrete model
custom_agent_definition	No task.model; a user-defined custom agent's definition supplied its model
unset	Neither the task call nor structured configuration supplied a model

The field is optional. Task-planned launches (including factory agents) populate it; other launch paths leave it absent.

Dependency status — September 8, 2026

• Runtime PR github/copilot-agent-runtime#18727 merged on September 5.
• CLI 1.0.84-2 is published. Its release schema was checked directly: it contains the optional field and all four enum values.
• Update Copilot CLI to 1.0.84-2 #2572 already regenerates all SDK bindings from this release, including the Node field and enum proposed here. This hand-edited PR will be redundant once Update Copilot CLI to 1.0.84-2 #2572 lands; prefer that generated update rather than duplicating the CLI version bump here.
• SDK main is still pinned to CLI 1.0.83, so the codegen check on this standalone hand-edit is not expected to pass. Closed at the author's request as superseded by Update Copilot CLI to 1.0.84-2 #2572; that generated update remains the dependency for the VS Code consumer.
• Companion agentHost: report where a subagent's task model came from microsoft/vscode#334184 still needs a published SDK containing the generated field before its dependency bump and temporary cast removal.

Original validation

• tsc --noEmit
• vitest run session-event (26 tests)
• prettier --check on the generated file

[bhavyaus]

Superseded by #2572, which updates the SDK to published CLI 1.0.84-2 and regenerates the bindings, including SubagentTaskModelSource and the optional SubagentStartedData.taskModelSource field proposed here. Closing this hand-edited PR in favor of that generated update. The VS Code consumer remains tracked in microsoft/vscode#334184.