V2: clean up session event schema names and payloads before beta #35014

Summary

Clean up the V2 session event schema before beta based on the live gang-grill review.

Source discussion: https://discord.com/channels/1391832426048651334/1522263729318658181

Locked decisions

• Drop session.next.*; next was migration scaffolding, not domain language.
• Event creation time belongs on the event envelope (created), not repeated as payload timestamp on ordinary events.
• Keep durability explicit in event definitions. Current durable naming is fine; make the durable/ephemeral split clear at the definition/API level.
• Remove projection-only messageID fields from events when projected message IDs can be derived from event IDs.
• Use session.agent.selected, session.model.selected, and session.location.selected.
• Rename location subdirectory to subpath to match Session.subpath; keep it on the event so projectors do not need filesystem/project resolution.
• Keep session.prompt.admitted / session.prompt.promoted; promoted should reference admitted input by inputID and must not duplicate the prompt payload.
• Leave legacy session.prompted compatibility alone for now; remove old cruft in a later boring cleanup release once V2 is stable.

Proposed cleaned shapes

session.prompt.admitted { inputID, prompt, delivery }
session.prompt.promoted { inputID }
session.agent.selected { agent }
session.model.selected { model }
session.location.selected { location, subpath? }
session.renamed { title }
session.forked { parentID, from? }

Follow-up checks

• Decide how deterministic projected IDs are derived from event IDs.
• Confirm which event payload IDs remain domain-significant (callID, from, etc.) vs projection leakage.
• Update generated schema/client/plugin APIs together.