Enforce warning-free Swift builds

[robinebers]

TL;DR

The app and test targets now treat every Swift warning as an error. All existing warnings are resolved, including actor-isolated reset presentation logic that has been moved out of the SwiftUI view into a pure model.

What was happening

• SwiftPM builds completed with recurring compiler warnings in provider defaults, cache bookkeeping, a shared formatter, and concurrency-sensitive tests.
• Reset formatting lived inside a SwiftUI View, which implicitly isolated otherwise pure helpers to the main actor.
• Nothing prevented a new warning from accumulating unnoticed.

What this changes

• Enables .treatAllWarnings(as: .error) for both the OpenUsage executable and test target.
• Documents the warning-free build and CI policy in AGENTS.md.
• Removes redundant pricing-closure awaits and the obsolete unsafe formatter annotation.
• Explicitly discards the cache set-insertion result.
• Makes the login-shell concurrency test synchronize its captured result.
• Extracts RateLimitResetsPresentation as a Foundation-only, independently tested model and keeps the view render-only.

Heads-up

• This intentionally makes future warnings CI-blocking. Dependency targets are not changed by the package target settings.
• The reset-presentation extraction has no intended visual or copy change.

Tests

• Focused warning and reset tests — 24 passed
• swift test — 962 XCTest tests passed, 3 skipped; 3 Swift Testing tests passed
• swift build — passed with warnings treated as errors
• ./script/build_and_run.sh verify — warning-free release build, signing, launch, and process verification passed
• git diff --check

---

Note

Low Risk
Mostly build policy and warning fixes plus a refactor with existing tests; behavior should be unchanged aside from stricter future builds.

Overview
Swift warnings are now CI-blocking for both the OpenUsage app and test targets via .treatAllWarnings(as: .error) in Package.swift, with AGENTS.md updated to match.

Compiler hygiene: default provider pricing closures drop redundant await on synchronous ModelPricingStore.current(); CopilotUsageMapper drops nonisolated(unsafe) on its static DateFormatter; ProviderSnapshotCache explicitly discards the set-insert result; and the login-shell off-main-thread test uses OSAllocatedUnfairLock instead of capturing a mutable var across concurrency boundaries.

Rate-limit resets popover: timeline formatting and the empty / count-only / timeline state machine move from RateLimitResetsDetail into RateLimitResetsPresentation (Foundation-only, independently unit-tested); the SwiftUI view only renders. No intended UI or copy change.

^{Reviewed by Cursor Bugbot for commit 85ee7ef. Bugbot is set up for automated code reviews on this repo. Configure here.}

[robinebers]

Superseded by #949, which combines the warning gate with deterministic build/run verification in two review-separated commits. #949 preserves this PR’s warning cleanup and pure presentation extraction, has a 202-line total diff, and is fully green. This branch remains preserved; nothing was merged.