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.

packages/core/agent-core/README.md

@@ -16,7 +16,7 @@ This is the package to read to see **the whole plugin tree at once** — the tea
 @deepseek-ai/dsh-tools            tool registry + tools/pre-execute/post-execute
 @deepseek-ai/dsh-agent            agent registry + agent/* event vocabulary
 @deepseek-ai/dsh-invariants       dev-mode event-contract assertions
-@deepseek-ai/dsh-tool-bash        the model-facing bash/bash_output/bash_kill schemas
+@deepseek-ai/dsh-tool-bash        the model-facing bash schema (background runs register with ctx.tasks)
 @deepseek-ai/dsh-agent-loop       THE concrete loop (gets the forwarded `agents`)
                                   (dsh-system-prompt gets the forwarded `persona`)
 ```