fix(agent): requestAbort() must cascade to active RLM child runs

[rynfar]

Part of #22 (Meridian reliability umbrella). P1.

Problem

AgentSession.requestAbort() (packages/coding-agent/src/core/agent-session.ts) aborts retry, compaction, bash, refine, and agent.abort() — but does not cancel active RLM child runs. Only abort() calls _cancelActiveRlmChildRuns(). Every user-facing cancel path uses requestAbort(): in-process-agent-connection.ts (both call sites), daemon abort and abort_and_clear_queue, and interactive Ctrl-C. So when a user cancels a parent turn, in-flight RLM children keep running — holding proxy concurrency slots and consuming the Claude Max subscription until they finish or idle out. This is the Prime-side half of Meridian's documented "incomplete parent-to-child cancellation" failure mode; per-child cancel machinery already exists (cancelRlmChildRun, _cancelRlmChildRun) but nothing user-facing invokes it.

Proposed shape

• requestAbort() cancels (or explicitly quiesces, for children intentionally detached as background work) active RLM child runs, or a cascade option threaded from the user-facing call sites — decide the default deliberately: retained/background children may need to survive a parent turn abort, but interactively cancelled foreground work should not leave orphans.
• The fork's correlated prompt lifecycle (packages/coding-agent/src/core/prompt-lifecycle.ts, currently pre-delivery only via cancelPromptLifecycle) is the natural home for scoped, recursive post-delivery cancellation — it already owns per-prompt ownership and request fingerprints.
• Pairs with proxy-side work (umbrella Meridian reliability: make concurrent RLM subagents production-safe over Claude Max #22): once children carry a parent linkage in metadata.user_id, Meridian can also tear down child SDK subprocesses when the parent's request aborts.

Acceptance

• Ctrl-C / connection abort on a parent with running foreground RLM children terminates the children's provider streams (verify no further child requests hit a stub provider after the abort).
• Retained/background children behavior is explicitly specified and tested, not incidental.