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.
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