refactor(session): extract sharing orchestration

[kitlangton]

Summary

• extract session sharing and root-session auto-share policy into a new SessionShare layer above Session
• remove share and unshare orchestration from Session, keeping it closer to core session state/persistence
• have ShareNext clean up shares on Session.Event.Deleted and update routes/callers to use SessionShare
• add a v2 note for removing the dedicated session.init compatibility route in specs/v2/session.md

Testing

• bun typecheck
• bun turbo typecheck
• bun run test -- test/share/share-next.test.ts