fix(tasks): address task API review feedback

The public kill result used the awkward phrase already-terminal. Rename it to already-finished and keep the model-facing response aligned; not-alive would be inaccurate because a force-failed registry record can still correspond to orphaned producer work.

Task kinds were open strings even though producer namespaces are an extension point. Add the merge-extensible TaskKindMap and derived TaskKind, cover consumer declarations in task and bundle tests, and retain the runtime non-empty check for untyped callers.

With exactOptionalPropertyTypes, owner?: Agent | undefined allowed an explicit undefined value that no caller needs. Tighten the property to owner?: Agent so unowned work is expressed by omitting it.

Record the requested task-service/backend split as a follow-up, using a systemd-backed runtime as a concrete candidate without guessing its durability and ownership contract in this PR. Regenerate the type and Cordis catalogs so public docs match the declarations.

docs/cordis-catalog/services.md

@@ -251,7 +251,7 @@ start(spec: TaskStart): TaskId
 list(caller?: Agent): TaskSnapshot[]
 get(id: TaskId, caller?: Agent): TaskSnapshot
 read(id: TaskId, caller?: Agent): TaskRead
-kill(id: TaskId, caller?: Agent, reason?: string): 'requested' | 'already-terminal'
+kill(id: TaskId, caller?: Agent, reason?: string): 'requested' | 'already-finished'
 async wait(id: TaskId, timeoutMs: number, caller?: Agent, signal?: AbortSignal): Promise<TaskSnapshot>
 onTaskDone(listener: TaskDoneListener): () => void
 attachSurface(name: string): () => void
@@ -259,7 +259,7 @@ attachSurface(name: string): () => void
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/tasks/tasks/src/index.ts:72`](../../packages/tasks/tasks/src/index.ts)
+Source: [`packages/tasks/tasks/src/index.ts:76`](../../packages/tasks/tasks/src/index.ts)
 
 ## `ctx.tools` — `ToolRegistry`