Refactor agent detail fetches onto useApi

[niufuti-cyber]

Summary

• Migrates the agent detail page's primary usage request to the shared useApi hook, keyed by the URL-encoded agent id so navigation aborts superseded usage requests.
• Keeps the optional lifetime-total request as a soft failure while guarding it so stale responses from a previous agent cannot overwrite the current view.
• Adds focused tests for success, usage errors, optional-total failure, rapid agent switching, unmount cancellation, and the updated shared-hook abort behavior.
• Documents the agent detail cancellation/stale-response behavior in the README.

Addresses #178

Verification

• npm run lint: passes with 0 errors and 3 pre-existing warnings in src/components/__tests__/ConfirmDialog.test.tsx
• npm run typecheck: passes
• npm test -- --runInBand --runTestsByPath src/app/agents/[agent]/page.test.tsx src/app/changelog/page.test.tsx: 2 suites passed, 10 tests passed

Notes

I am not claiming the full suite or build are currently green:

• Full npm test -- --runInBand was checked against the same base and has unrelated baseline failures in src/app/usage/page.test.tsx and src/components/__tests__/ConfirmDialog.test.tsx.
• npm run build was checked against the same base in a no-network verification environment and fails before this patch because the app imports Google fonts through next/font/google.

[mikewheeleer]

thanks for the effort @niufuti-cyber! 🙏 i can't tie this to an issue assigned to you, and we merge from the assigned contributor to keep the campaign fair. please claim an open unassigned issue first, then open your PR. closing for now — hope to see it back 🙌

[mikewheeleer]

great stuff — thanks for this!