Allow public GitHub issue reads without token

[cybercraftsolutionsllc]

Fixes #743.

Summary

• Allow the GitHub API client to fall back to unauthenticated requests when GITHUB_TOKEN is not configured.
• Keep bearer-token behavior unchanged when a token is configured.
• Add focused tests for both empty-token public issue reads and configured-token Authorization behavior.

Why

Public GitHub issue reads should not require a configured token. The old client always wrapped requests in OAuth even when GITHUB_TOKEN was empty, which can cause GitHub to reject public issue lookups as bad credentials and surface as "make sure the repo is public" style failures.

Validation

• go test handlers/github.go handlers/github_test.go
• git diff --check

Note: go test ./handlers -run 'TestGithubClientWithToken|TestGetGithubIssue' is currently blocked by existing generated mock drift in unrelated handler tests: mocks.Database is missing GetBountyByUnlockCode.