chore: update keyring after read-error fix is released

[skvark]

Context

The CLI currently resolves @napi-rs/keyring@1.3.0. That version converts both missing keychain entries and platform/access failures to null, so downstream code cannot distinguish an unauthenticated user from an unavailable keychain.

The upstream fix is Brooooooklyn/keyring-node#136. The downstream error-propagation preparation is githits-com/githits-cli#240.

Follow-up

• confirm the upstream PR is merged and a fixed npm release is available
• bump @napi-rs/keyring to the fixed release
• update bun.lock and verify every resolved platform package uses the fixed version
• run the auth storage tests, full suite, build, and CLI/MCP smokes
• validate missing-entry and keychain-access-failure behavior on macOS
• remove the temporary 1.3.0 limitation note from docs/implementation/auth.md

Until the dependency is updated, PR #240 is safe to merge but does not resolve the reported macOS behavior for installed users.