[0.2.1] subagents service removed start(provider, request) in favor of startActivation(spec) without a deprecation window — third-party plugins break at registration

[1225Sakura]

What happened. Between 0.2.0-rc.2 and 0.2.1-rc.5, the subagents cordis service (@deepseek-ai/dsh-subagent, SubagentRuntime) dropped the service-level start(provider, request) method and replaced it with startActivation({ provider, label, request, signal, delivery }). The first-party dsh-tool-subagent was migrated, but third-party plugins calling ctx.subagents.start(...) fail at registration time with no migration notice.

Impact. Any plugin with a dispatch integration (ours: oh-my-deepseekharness omd_delegate) degrades from working to registration failure. Because the service is still present and visible, feature detection reports "service exists but shape incompatible" — a confusing failure mode that took three review rounds to root-cause.

Evidence.

• 0.2.0-rc.2: SubagentRuntime.prototype includes start (our integration worked — reached provider layer).
• 0.2.1-rc.5: service surface = startActivation/getProvider/list/expectProvider/resolveMaxDepth/…, no start; start survives only on provider objects (optional, types/types.d.ts:326).
• Migration cost on our side: dual-shape feature-detect + new call surface (delivery semantics, parent identity check, resolveMaxDepth, content-block prompt) + dual contract tests.

Requests (either helps).

1. Keep start(provider, request) as a deprecated alias for one minor cycle, delegating to startActivation; or
2. Document the break prominently in the 0.2.1 changelog/release notes with a migration snippet (old call → new spec).

Environment. dsh 0.2.1-rc.5 (npm latest 2026-11-02), profile=web, Windows 11, Node 22.