chore: pin rust-toolchain to 1.98.0 to match CI's stable

[getappz]

Summary

Opened by claude for item #178 via agentflare.

Local stable (1.97.0) and CI's stable (1.98.0) produced different rustfmt output for nested match expressions — hit while shipping PR #585, causing repeated CI-only fmt failures with no way to reproduce locally.

Adds rust-toolchain.toml pinning to 1.98.0. dtolnay/rust-toolchain@stable (used in .github/workflows/ci.yml) auto-detects this file when no explicit toolchain input is given, so no CI workflow change is needed — verified this repo's usage matches that (no explicit toolchain: input in either the clippy or fmt job).

Test plan

• Verified locally: rustc --version inside the repo now resolves to 1.98.0 automatically via the pinned file
• cargo fmt --check and cargo check --workspace clean under the pinned toolchain

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• rust-toolchain.toml

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