feat(tasks): background task runtime, generic task_* control tools, bash/subagent producers

One shared ctx.tasks registry (branded <kind>-N ids, owner-fenced
read/kill/wait/list, attachSurface misconfiguration fence, reported-flag
notice dedup, atomic register) + dsh-tool-tasks (task_output/task_list/
task_kill, completion-notice injection, background prompt habit).
Producers opt in via their own enableRunInBackground config: bash
(stream kind; seam slimmed to resolve/run/start returning a BashProcess
handle, bash_output/bash_kill deleted) and subagent (final-output kind;
done settles after run.dispose()). Owner disposal drains tasks through
the new awaited ctx.agents.onCleanup seam in the loop's disposal chain.
Both RFCs moved to implemented/; docs, catalogs, snapshots re-pinned.

scripts/gen-doc-graphs.ts

@@ -73,6 +73,7 @@ const GROUP_ORDER = [
   'fs',
   'compact',
   'subagent',
+  'tasks',
   'web',
   'todo',
   'hooks',
@@ -186,6 +187,14 @@ const SERVICE_ROLES: ServiceRole[] = [
     consumers: ['tool-subagent'],
     note: 'Providers implement transports; tool-subagent exposes one configured provider as a model-facing tool name.',
   },
+  {
+    key: 'tasks',
+    pkg: 'tasks',
+    title: 'Background task registry',
+    mode: 'core',
+    consumers: ['tool-bash', 'tool-subagent', 'tool-tasks'],
+    note: 'Producers (tool-bash background commands, tool-subagent background delegations) register running work; tool-tasks is the model-facing control surface that reads, lists, and kills it.',
+  },
   {
     key: 'web',
     pkg: 'web',