contextual slog sender doesn't support phase runs

[mhofman]

Describe the bug

Vat termination formalized the concept of run phases, and added a phase attribute to cosmic-swingset-run-* events. It also added a new cleanup phase, that generates potential swingset work but does not create a new "trigger" slog event unlike the inbound or timer triggers.

To Reproduce

Observe slogs on u18, and notice the "run 2" entries have an unknown trigger

Expected behavior

Using the new phase attribute to help classifying the trigger:

• Right now we assume run=0 is always a leftover phase. In the future we may decide to skip the run altogether if we know there is no work to do. Instead we should only continue / restore a previous trigger info if we're in a leftover phase.
• For the cleanup phase, correctly populate the trigger type.

Testing consideration

Manual for now. In the future we'll need something like #10365 and the ability to conditionally error when encountering unknown triggers