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.

packages/examples/agent-spine-demo/README.md

@@ -22,7 +22,7 @@ Read this package for the whole plugin tree and its composition order.
 @deepseek-ai/dsh-tool-goal        optional model-facing goal controls
 @deepseek-ai/dsh-goal-session     optional same-session goal-round driver
 @deepseek-ai/dsh-llm-retry        bounded transient request retry policy
-@deepseek-ai/dsh-tasks            generic background-task registry
+@deepseek-ai/dsh-tasks-local      generic background-task registry
 @deepseek-ai/dsh-invariants       configurable invariant registry service
 @deepseek-ai/dsh-session/invariant
 @deepseek-ai/dsh-agent/invariant