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/gen-doc-graphs.ts

@@ -365,9 +365,10 @@ const SERVICE_ROLES: ServiceRole[] = [
     key: 'tasks',
     pkg: 'tasks',
     title: 'Background task registry',
-    mode: 'core',
+    mode: 'seam',
+    implementations: ['tasks-local'],
     consumers: ['tool-bash', 'tool-pty', 'tool-subagent', 'tool-tasks'],
-    note: 'Producers (background bash, PTY sends, and subagent delegations) register running work; tool-tasks is the model-facing control surface that reads, lists, and kills it.',
+    note: 'Producers (background bash, PTY sends, and subagent delegations) register running work; tool-tasks is the model-facing control surface that reads, lists, and kills it; tasks-local is the process-local registry.',
   },
   {
     key: 'web',