feat(plugin-sdk): thread agentId into environmentAcquireLease params

Add an optional agentId field to PluginEnvironmentAcquireLeaseParams and
thread it through the host's environment-runtime + run-orchestrator call
sites so plugin-backed sandbox providers can scope lease state (subdirs,
PVCs, etc.) per agent without an SDK callback or DB lookup.

The field is required-but-nullable on the internal EnvironmentDriverAcquireInput
(string | null) so every call site has to think about whether it has an
agent context. Ad-hoc operator probes (agent test-environment route)
pass null. The plugin RPC payload omits the field entirely when null,
keeping wire compatibility with older plugin worker SDKs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/plugins/sdk/src/protocol.ts

@@ -379,6 +379,13 @@ export interface PluginEnvironmentLease {
 
 export interface PluginEnvironmentAcquireLeaseParams extends PluginEnvironmentDriverBaseParams {
   runId: string;
+  /**
+   * UUID of the agent the run is being acquired for. Omitted only for ad-hoc
+   * invocations (e.g. operator-initiated environment test probes) where no
+   * agent context exists. Plugins should treat undefined as "no per-agent
+   * partitioning available" and fall back to environment-level behavior.
+   */
+  agentId?: string;
   workspaceMode?: string;
   requestedCwd?: string;
 }