fix: surface fallback/quota failures to prevent opencode from hanging indefinitely

[win0na]

Summary

• fixes Show quota limit #115 & Hangs when limits have been reached #168
• surface generic 429, 503, and 529 Anthropic API failures immediately so upstream fallback can take over instead of the plugin masking those failures and hanging behind its own retry path
• keep plugin-owned recovery bounded by retrying a 401 credential refresh only once, adding request timeouts for hung outbound auth and metadata fetches, and aborting stalled successful SSE streams after the idle timeout
• add and update coverage for fallback handling, request timeout behavior, and stalled stream handling, and update the README plus changelog to document the timeout and fallback behavior

Related issue

N/A.

Testing

• make all
• CI=true pnpm test
• pnpm test
• pnpm run build
• CI=true pnpm exec node --test --experimental-strip-types --test-name-pattern "auth loader aborts hung outbound fetch within request timeout|auth loader preserves caller abort signal over request timeout" src/index.test.ts
• node --test --experimental-strip-types src/index.test.ts --test-name-pattern "auth loader surfaces generic 429 immediately for upstream fallback|auth loader surfaces 503 immediately for upstream fallback|auth loader surfaces 529 immediately for upstream fallback|auth loader retries once on 401 before surfacing terminal failure"
• node --test --experimental-strip-types src/transforms.test.ts --test-name-pattern "transformResponseStream errors stalled SSE stream after idle timeout|transformResponseStream continues streaming when boundaries arrive before timeout|transformResponseStream passes 429 errors through intact"

Checklist

• PR title follows Conventional Commits (feat:, fix:, docs:, chore:, etc.)
• make all passes locally (runs lint, build, and test)
• Tests added or updated where applicable
• README or docs updated where applicable

[win0na]

The above commits fix the linting & CI build issues that were present, & update the CHANGELOG.md to reflect the latest version.

[tim-hilde]

Would solve #115 and #168