release: require Rust 1.98 in Unpin 1.4.3

[IgorArkhipov]

Summary

Unpin 1.4.3 now uses Rust 1.98 across local development, CI, release automation, and Trunk. Source builds with Rust 1.96 or 1.97 are no longer supported; prebuilt CLI and desktop behavior and the release artifact target set are unchanged.

Test Plan

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo test --workspace --all-features --locked
• cargo run -p unpin-cli --locked -- --help
• cargo audit --deny warnings
• cargo deny check
• cargo machete
• xcodebuild test -project apps/unpin-desktop/UnpinDesktop.xcodeproj -scheme UnpinDesktop -destination 'platform=macOS'
• Pi/OpenCode disposable-root host validation (providerStateUnchanged: true)
• Full local provider matrix finalized on the exact merged main commit before tagging

Safety Checklist

• Tests and examples do not read real home-directory provider state.
• Tests and examples do not use .env* files.
• Write-path safety behavior is unchanged.
• Provider support remains within the current Claude Code, Codex CLI, Cursor, Pi, OpenCode, and Zed scope.
• Public source-build requirements and current-version install examples are updated.

Reviewer Notes

Please verify the Rust-version consistency check covers every active toolchain surface and that the 1.4.3 package, desktop, documentation, and release-note versions agree. The full provider matrix and post-tag artifact verification remain publication gates.