Expose per-session cost to hooks / TUI (or add the a cost session section in the TUI)

Feature Description

Today hooks (PreToolUse/PostToolUse/Stop/SessionStart) receive a stdin payload with tool_input/tool_response, but it does not include the
LLM usage (input/output/cache tokens) or the model per turn. Also, the session transcript JSONL
(~/.commandcode/projects//.jsonl) persists only the message content, not the usage.

I know CommandCode already computes usage internally (it powers the "updated" indicator in the TUI) and exposes /internal/usage/summary at
the account level, but there's no way to see the accumulated cost of the current session from outside.

Request: include usage (input_tokens, output_tokens, cache_read_input_tokens, cache_creation_input_tokens) and model per turn in the hook
payload, and/or persist that usage in the transcript JSONL. That would let a Stop/SessionStart hook display the session cost in the TUI via
systemMessage without patching the dist. As an alternative, an endpoint like /internal/usage/summary?sessionId= would return per-session
cost directly. Or add a cost session section in the TUI.

Would this be something you'd consider for upcoming hook events?

Use Case

No response

Additional Context

As a developer who runs long, multi-turn coding sessions with Command Code, I want to see the running cost of the current session directly
in the TUI so I can make informed decisions without leaving the terminal — for example, stopping or switching models when a session is
getting expensive, comparing cost across models (non-open vs. open models) on the same task, or tracking spend per project/client for billing.

Today the only cost visibility is the account-level /internal/usage/summary (after the fact, mixed across all sessions) or the internal
"updated" indicator, neither of which tells me what the session I'm actively in is costing. If per-turn usage and model were exposed to
hooks (or persisted in the transcript), I could write a small Stop/SessionStart hook that prints the live session cost as a systemMessage in
the feed — no dist patching, fully under my control.

How important is this to you?

Ver important.

Start by tracing the hook payloads for PreToolUse, PostToolUse, Stop, and SessionStart, then inspect the session transcript JSONL and the existing /internal/usage/summary path mentioned in the issue. Determine whether per-turn usage and model data can be exposed through hooks, persisted in transcripts, or returned for a session; done means one supported way to obtain current-session cost for TUI display.