[FEATURE] Show total time and token/cost usage for each finished session

Problem Statement

There is no per-session accounting anywhere in Claude Code. /usage reports the active 5-hour block and totals by period, which answers "how much have I used lately" but never "what did that session cost".

That gap matters as soon as sessions get long. Two sessions that look the same in the list can differ by an order of magnitude in tokens, because cache reads scale with how large the context has grown and how many turns were taken at that size. Without a per-session figure there is no way to compare two ways of doing the same work, to notice afterwards that one session ate a large share of the weekly allowance, or to tell which part of a workflow is expensive. Measuring it today means parsing the session's own .jsonl transcript and summing message.usage by request id — which works, but nobody should have to write that script.

Proposed Solution

When a session ends, show a compact summary for that session:

• total wall-clock time the session ran
• total tokens, ideally split into input / output / cache-creation / cache-read
• the API-equivalent cost of those tokens

Put it in two places: as a line under each finished session in the session list, and at the end of the transcript when the session closes. The single API-equivalent cost figure is the important part; the token breakdown is a bonus.

Alternative Solutions

Summing message.usage from the transcript JSONL, collapsing records by requestId so the per-content-block duplicates are not double counted. It gives the right number but requires knowing the transcript format, and it is not available to anyone who is not scripting against it.

/usage covers aggregate consumption and is not a substitute: it cannot attribute usage to one session.

Additional Context

Applies to Claude Code both in the desktop app and in the CLI. With auto-compaction disabled and long-running sessions, the per-session figure is the one number that makes cost decisions possible.