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/doc-budgets.manifest.json

@@ -1,11 +1,11 @@
 {
   "AGENTS.md": 1691,
   "docs/AGENTS.md": 1315,
-  "docs/architecture.md": 1640,
+  "docs/architecture.md": 1760,
   "docs/cordis-primer.md": 550,
   "docs/defensive-patterns.md": 550,
   "docs/testing.md": 800,
   "examples/AGENTS.md": 610,
   "packages/AGENTS.md": 450,
-  "packages/README.md": 605
+  "packages/README.md": 645
 }