ref(hookStore): Change HookStore to single-value semantics

[evanpurkhiser]

HookStore.get() previously returned an array of registered callbacks,
but in practice only a single callback was ever registered per hook
name. The registration pattern (object literal keys in GETSENTRY_HOOKS,
and the remove-then-add pattern in GSBillingNavigationConfig) already
guaranteed at most one callback per slot.

Tighten the types to reflect reality:

• get() returns Hooks[H] | undefined instead of Array<Hooks[H]>
• add() is renamed to set() and overwrites instead of pushing
• remove() no longer takes a callback argument — it just clears the slot

All consumers updated: [0] indexing gone, .forEach/.map loops replaced
with single optional calls.

[github-actions]

📊 Type Coverage Diff

Metric	Before	After	Delta
Coverage	93.56%	93.56%	±0%
Typed	135,565	135,552	🔴 -13
Untyped	9,338	9,335	🟢 -3

🔍 2 new type safety issues introduced

any-typed symbols (1 new)

File	Line	Detail
static/app/components/hook.tsx	60	HookComp (var)

Type assertions (as) (1 new)

File	Line	Detail
static/app/components/hook.tsx	60	as React.ComponentType<any> — hookCallback as React.ComponentType<any>

This is informational only and does not block the PR.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 0bd87e0. Configure here.}