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.

docs/rfc/rejected/simplification/2026-06-20-drop-bash-output-spill-files.md

@@ -12,7 +12,7 @@ This solves a real problem, but in a narrow and leaky way. A spill path is a pro
 
 Keep tail truncation, drop full-output spill files. A bash result contains the bounded tail plus a clear truncation marker; no path is emitted. If users need full-output recovery, add a generic artifact/blob service with explicit ownership, cleanup, and UI rendering, then let bash attach large outputs to that service.
 
-This proposal can land independently of [a generic long-running tool runtime](../../proposed/architecture/2026-06-20-generic-long-running-tool-runtime.md). If background tasks stay, `bash_output` should still report that output was dropped, but without advertising a spill path.
+This proposal can land independently of [a generic long-running tool runtime](../../implemented/architecture/2026-06-20-generic-long-running-tool-runtime.md). If background tasks stay, `bash_output` should still report that output was dropped, but without advertising a spill path.
 
 ## Acceptance criteria