fix: skip Claude usage lookup for non-Claude agents

[catwithtudou]

Summary

• Skip Claude usage keychain lookup when the active agent is not Claude
• Return an unavailable state from the Claude usage API for non-Claude agents
• Render that unavailable state in the Usage page and cover both Claude and non-Claude route branches with tests
• Add TSDoc for the new usage route options and usage response types

Test Plan

• bun test
• bun run check
• bun run build:js
• bun run build:bin