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.

examples/coding-agent/cordis.yml

@@ -2,7 +2,8 @@
 # backends — the DeepSeek adapter and the local bash executor — plus `hmr` for
 # the dev/demo reload loop, then the stdio chat app (@deepseek-ai/dsh-stdio-
 # agent), which bundles the whole agent-core spine (timer, llm, sessions,
-# system-prompt, tools, agents, invariants, tool-bash, agent-loop), the console
+# system-prompt, tools, agents, tasks + the task_* control tools, invariants,
+# tool-bash, agent-loop), the console
 # logger, JSONL persistence, the readline UI, and a pre-created `main` agent.
 #
 # `hmr` is a leaf entry (not baked into dsh-stdio-agent): it is a Loader-only