feat: Session cost display and budget warning notifications

[rdwj]

Summary

Show users their token consumption and estimated cost for the current session. Display warnings when approaching budget limits so users can make informed decisions about continued interaction.

UX Requirements

• Token usage display in session header or sidebar (input tokens, output tokens, estimated cost)
• Budget progress bar when budget limits are configured
• Warning notification (toast or banner) when approaching soft budget limit (e.g., 80%)
• Error/blocking notification when hard budget limit is reached
• Per-message token count available on hover or in message metadata popover
• Cost breakdown by model if multiple models are used in the session
• Usage data refreshes after each assistant response, not on a polling interval
• Dismissible warnings — don't nag after acknowledgment

Implementation Notes

Read usage data from GET /v1/sessions/{id}/usage via the gateway. Budget information can also arrive in response headers (X-Token-Usage, X-Budget-Remaining) on each chat completion response. The UI should cache the latest usage snapshot and update incrementally. Budget thresholds are configured server-side; the UI only renders them.

Companion Issues

Companion issues filed on fips-agents/agent-template, fips-agents/gateway-template, fips-agents/fips-agents-cli, and fips-agents/examples.

Size

S-M