fix(gateway): make bg-process reset TTL configurable + surface session-scoped processes

Follow-up to the cherry-picked #29212 (#29177):

- Promote the 24h stale-process threshold to config.yaml
  (session_reset.bg_process_max_age_hours) instead of a hardcoded
  constant. 0 disables the cutoff (legacy: any live process blocks reset).
  Wired through GatewayConfig.default_reset_policy in gateway/run.py.
- Bug 2: process(action=list) now resolves the gateway session_key from
  the contextvar and surfaces session-scoped background processes (a
  forgotten preview server under a different task), flagged
  session_scoped — so the agent/user can discover and kill the blocker.
  Previously the task-scoped list returned [] and the blocker was invisible.
- Tests: config round-trip for the new field, cross-task list visibility.
- Docs: messaging session-reset section.

website/docs/user-guide/messaging/index.md

@@ -198,6 +198,15 @@ Sessions reset based on configurable policies:
 | Idle | 1440 min | Reset after N minutes of inactivity |
 | Both | (combined) | Whichever triggers first |
 
+A live background process (started with `terminal(background=true)`) normally
+protects its session from resetting so output isn't lost. To stop a forgotten
+process — say a preview server — from pinning a session open forever, a
+background process older than `bg_process_max_age_hours` (default **24**) no
+longer blocks reset. The process is **not** killed, only ignored by the reset
+guard. Set it to `0` to disable the cutoff (any live process blocks reset, the
+old behavior), or raise it if you run legitimate multi-day jobs whose liveness
+should keep the conversation open.
+
 Configure per-platform overrides in `~/.hermes/gateway.json`:
 
 ```json