refactor(job-controller): drop the subagent ownership fence from job.kill

The registry's owner fence is the only access rule for a human kill: a job
is killable by the session whose list carries it, and a child session's own
jobs are no exception. The fence had pulled two implementation helpers out
of the Session Controller's package entry (plus a safe-host-export exception
for them) so the job controller could import them; both go, and the job
controller no longer depends on the agent registry or the Session Controller.

packages/api/session-controller/src/index.ts

@@ -63,7 +63,7 @@ import type {
 } from './types.ts'
 
 export type * from './types.ts'
-export { ApiSessionNotFound, apiSessionSubagentOwnershipError, hasApiSessionSubagentOwner } from './agent.ts'
+export { ApiSessionNotFound } from './agent.ts'
 export { SessionFileReferences } from './file-references.ts'
 export { SessionSkillCatalog } from './skill-catalog.ts'