Correct stale guidance about mobile navbar ownership

## Problem

The wallet has rendered its persistent bottom navigation through the React `BottomNav` since commit `5fd95811`, including in the Capacitor mobile app. However, `AGENTS.md`, `CLAUDE.md`, and the shared frontend skill still describe the retired Swift/Android navbar overlay as the active mobile implementation.

That stale guidance already affected issue #802 by asking contributors to update both a native navbar and a React footer even though only the React path is reachable.

## Scope

- Update `AGENTS.md` and `CLAUDE.md` to describe the current React-owned navigation architecture.
- Remove references to the deleted `DappBrowserProvider` navbar wiring and `use-native-navbar-action` test hook.
- Update `skills/miden-wallet-frontend/` so its platform and verification guidance points contributors to `BottomNav`, `TabLayout`, safe-area behavior, and mobile interaction requirements.
- Preserve historical changelog entries as historical records.
- Keep the canonical skill and both agent links consistent.

## Acceptance criteria

- [ ] Current repository instructions no longer claim that the mobile navbar is a native overlay.
- [ ] The frontend skill identifies React `BottomNav` as the shared navbar across extension, mobile, and desktop.
- [ ] Guidance still covers mobile safe areas, haptics, and platform-specific verification.
- [ ] No current instruction references deleted navbar wiring or hooks.
- [ ] Skill validation and `git diff --check` pass.

Found while validating #802 in PR #807.