refactor(tasks): split the task registry into seam and local implementation

The tasks/ family now matches the capability-seam shape: @deepseek-ai/dsh-tasks
keeps the abstract TaskService (ctx.tasks contract, vocabulary types, snapshot
invariant companion) and the new @deepseek-ai/dsh-tasks-local carries the
process-local registry (LocalTaskService: in-memory store, settlement,
owner-cleanup effects, teardown, TASK_WAIT_TIMEOUT). Compositions and test
harnesses now load dsh-tasks-local; producers, TaskKindMap merges, and
dsh-tool-tasks keep importing the seam only.

Producer misconfiguration diagnostics name dsh-tasks-local because loading the
implementation is the fix. The registry behavior suite moves to tasks-local;
the seam keeps a stub-subclass registration test and the probe-based invariant
suite.

scripts/verify-package-readme-model-experience.ts

@@ -93,6 +93,7 @@ const SENTENCE_MODEL_EXPERIENCE: Readonly<Record<string, SentenceContract>> = {
   'packages/support/llm-mock-server': { kind: 'none', reason: 'The test server substitutes provider wire behavior without invoking a real model.' },
   'packages/support/llm-replay': { kind: 'none', reason: 'The keyless adapter invokes no provider model.' },
   'packages/tasks/tasks': { kind: 'indirect', reason: 'Producer and control-surface plugins own all model rendering over the task registry.' },
+  'packages/tasks/tasks-local': { kind: 'indirect', reason: 'The registry backend delegates model rendering to producer plugins and dsh-tool-tasks.' },
   'packages/examples/acp-demo': { kind: 'indirect', reason: 'The app bundle delegates request composition to dsh-agent-spine-demo and dsh-acp.' },
   'packages/ui/app-boot': { kind: 'indirect', reason: 'Only the loaded plugin tree contributes model context.' },
   'packages/examples/jsonrpc-demo': { kind: 'indirect', reason: 'Only the externally configured plugin tree contributes model context.' },