Show per-subagent AI credit usage breakdown in /usage

Describe the feature or problem you'd like to solve

Copilot CLI displays cumulative AI-credit usage for the session, including 4 usage by the main agent, subagents, and background operations. However, it 5 does not show how those credits are distributed among individual agent 6 invocations. 7   8 This makes it difficult to identify expensive agents, compare custom-agent 9 models, and optimize multi-agent workflows.

Proposed solution

Extend /usage with an optional itemized breakdown showing:

• Main-agent usage
• Each subagent invocation
• Custom-agent name
• Model used
• AI credits consumed
• Background operations such as context compaction
• Total session usage

Example prompts or workflows

Use case

When a parent agent delegates investigation, implementation, review, and
testing to several subagents, users need to understand which invocation or
configured model accounts for most of the session cost.

Example

Main agent                         4.20 AIC
Explore #1 — architecture          1.15 AIC
ios-implementer #1                 7.80 AIC
Task #1 — tests                    0.65 AIC
Context compaction                 0.30 AIC
--------------------------------------------
Session total                     14.10 AIC

Possible commands:

/usage
/usage --by-agent
/usage --detailed

Additional context

No response

Empieza por el comando /usage y la contabilidad existente de créditos de sesión descrita en el issue. Traza cómo se registra el uso de main-agent, subagent, custom-agent, model y background-operation, y luego define el desglose mostrado y los totales para las formas de comando propuestas. Se considera terminado cuando /usage pueda mostrar detalles por invocación y conserve el total acumulado de la sesión.