[Feature] Per-conversation token and cost accounting

[HCH-hash]

Summary

Record and surface token usage and estimated cost per agent conversation so teams can monitor spend, quotas, and rate-limit pressure.

Problem

Nothing currently records token or cost data on the agent or conversation entities. For ACP agents the CLI transcripts often contain usage information, but the bridge does not forward it. Teams have no native way to see cost or to pace against rate limits.

Proposed Direction

• Capture usage (input/output tokens, and where available cost) from LLM providers and from ACP bridge transcripts.
• Persist the data on the conversation (or a related usage table).
• Expose it in the UI (conversation detail, list aggregates, project-level cost views) and via API.

Scope

• Product behavior
• Repository structure
• Architecture
• Developer experience
• Documentation
• Other

Additional Context

• Enables cost/quota dashboards and helps teams stay under rate limits.
• ACP path requires the bridge to forward usage; pure LLM agents can obtain it more directly from the provider response.
• Alternative: start with tokens only and add cost estimation later.

[pullfrog]

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