[Bug] TUI does not display cumulative session cost

[Xsidz]

Problem

The TUI shows context usage in the tray but never shows cost. AgentConnectionState has no sessionCost field, so interactive-mode.ts cannot surface it even though the session tracks it internally via getSessionStats().

Fix

1. Add sessionCost?: number to AgentConnectionState in types.ts
2. In snapshot.ts, read session.getSessionStats() once and set both contextUsage and sessionCost from it
3. In interactive-mode.ts, add formatSessionCost() and update getTrayContextLabel() to display the cost alongside context usage

PR #919 implements this fix. Posting here per the new discussion-first process.