Migrate GhinClient public API to neverthrow Result types

[boorad]

Background

CodeRabbit flagged this on #41 (review comment) for the GPA surface, but the issue is class-wide: every public method on GhinClient returns Promise<T> and throws on error, which contradicts the repo's stated convention:

neverthrow Result types for error handling — functions return Result<T, E> instead of throwing
— CLAUDE.md

The underlying RequestClient already returns Result<T, E>; GhinClient unwraps it (throwing on isErr()) before handing values back to callers, discarding the typed-error guarantee at the public boundary.

Scope

All 7 public surfaces on GhinClient (~28 methods) in src/client/ghin/index.ts:

• courses — 5 methods (getCountries, getDetails, search, getTeeSetRating, getTeeSetRatingsForScorePosting)
• facilities — 1 method (search)
• golfers — 4 methods (getOne, getScores, search, globalSearch)
• gpa — 4 methods (getAccesses, requestAccess, updateStatus, revokeAccess)
• handicaps — 4 methods (getOne, getCoursePlayerHandicaps, getCourseHandicaps, getPlayingHandicaps)
• scores — 3 methods (postHoleByHole, postAdjusted, post18h9and9)
• webhooks — 7 methods (get, patch, delete, test, list, resend, ensureRegistered)

Each method signature changes from Promise<T> → Promise<Result<T, GhinError>> (or a more specific error union per surface).

Why all at once

Migrating only one surface (e.g. GPA) would leave an inconsistent public API where some methods throw and others return Results — strictly worse than the current uniform-but-wrong state. The change is mechanical but breaking, so it should land in a single release.

Out of scope

• Internal helpers and RequestClient already use Result; no changes needed there.
• Error-class hierarchy under src/errors/ is fine as-is; this is purely a wrapping change.

Acceptance criteria

• All public methods on GhinClient return Promise<Result<T, E>>
• Tests updated to assert on result.isOk() / result.isErr() instead of .rejects.toThrow(...)
• Playground scripts in src/playground/ updated to the new shape
• Changeset marks this as a major version bump (breaking)
• README / examples updated