feat(agent): add compact context meter with usage details

[DavidBabinec]

What changed

• replace the full-width context bar with a five-segment remaining-capacity meter beside the composer actions
• add a hover/focus tooltip with exact context, cumulative conversation token usage, authoritative spend, and current-model rates
• tag context snapshots to the model that measured them, leaving a switched model indeterminate until its next response
• make the shared Tooltip optionally focus-open, preserve existing descriptions, and consume Escape before parent panels
• expose server-priced terminal usage cost and share AI usage formatting between the composer and Audit

Why

This keeps the composer compact while making context headroom and billing detail accurate, keyboard-accessible, and safe across model switches.

Verification

• bun test
• bun run build
• bun run lint
• bunx react-doctor --verbose --diff origin/main --no-score