feat(sdk): expose active sessions (#33991)

specs/v2/session.md

@@ -25,6 +25,11 @@ sessions.interrupt(sessionID)
   -> clears a coalesced follow-up wake already registered with this coordinator
   -> preserves durable inbox rows for a later wake or resume
   -> idle or missing Session is a no-op
+
+sessions.active()
+  -> snapshots foreground Session drains owned by this process
+  -> returns only active Session IDs with { type: "running" }
+  -> absence means inactive; activity is not durable across process restarts
 ```
 
 `session_input` is the durable admission inbox. `PromptAdmitted` records and projects accepted input so pending queue state can be replayed, replicated, and observed by clients. Admitted inputs remain outside model-visible Session history until the serialized runner publishes `Prompted`. Its projector atomically writes the visible user message and marks the inbox row promoted in the same event transaction. The V1-to-V2 shadow bridge publishes the same `Prompted` event for already-visible V1 prompts.
@@ -161,6 +166,8 @@ Post-crash continuation recovery is intentionally deferred. A wake does not infe
 
 A process-global `SessionRunCoordinator` serializes execution for each local Session while allowing different Sessions to run concurrently. Resumes join active execution, overlapping wakes coalesce into one follow-up, and interruption stops current process-local execution without deleting durable inbox work. The runner enters the Session's current Location when execution starts and fences each new provider turn against that Location.
 
+The coordinator's active registry is also the source for `sessions.active()`. It represents only foreground Session drains owned by the current process; background subagents and tasks do not add parent Sessions to this registry. The snapshot is runtime state and is empty after a process restart.
+
 Inbox promotion coalesces pending steers in durable admission order. Once continuation would otherwise end, it promotes one queued input at a time in FIFO order. Add explicit inbox backlog and steering-batch limits before exposing broad multi-caller admission or untrusted queue growth.
 
 Eager local-tool execution is intentionally unbounded in the current local slice. This minimizes tool latency but does not increase SQLite settlement throughput: Session-event publication remains serialized per provider turn. Before broadening exposure, revisit per-turn call limits, output truncation, and operational backpressure using observed workloads. The `session.next.*` event schemas remain experimental and unshipped; databases created by earlier experimental builds are disposable rather than compatibility targets.