Scheduled tasks — context_from chaining (seed from a specified prior fire)

Summary

Add context_from: <fireID> to ScheduleSpec (or a fire-time override) so a fire can seed from a SPECIFIED prior fire's session instead of the immediate prior (LastFireSessionID). The Hermes Agent pattern.

Part of #189 (scheduled tasks). Split out of #236 (deferred/v2) — depends on the carried-context toggle landed in #256.

Scope

• A ContextFrom string field on ScheduleSpec (empty = use LastFireSessionID, the #256 default; non-empty = load the specified fire's session instead).
• The fire path loads the specified fire's session by id and renders it as a fenced untrusted preamble, reusing #256's renderCarriedContext helper. Fencing discipline unchanged — the specified fire's context is equally untrusted.
• Cross-schedule chaining: the specified fire may belong to a DIFFERENT schedule. Its session is loaded by id, no schedule join.
• ContextFrom + OneShotRetry interaction: same as CarryContext — a re-armed one-shot does NOT carry context on the retry.

References

• Design: #189, ADR 0059.
• The v2 finish: #256 (landed CarryContext + the fenced-preamble rendering this builds on).

Start at the ScheduleSpec definition and the scheduled-task fire path, then read the CarryContext implementation and renderCarriedContext helper from #256. Trace how fire sessions are loaded, including retry handling, and consult ADR 0059 and #189 for the intended chaining behavior. Done means ContextFrom selects a session by ID across schedules while preserving fenced rendering and OneShotRetry semantics.