build: pin Rust 1.98.1 for reproducible release builds

[rNoz]

Summary

• Pin repository-local Rust builds to stable Rust 1.98.1 with a minimal rustup profile and the formatting and linting components.
• Align CI, Bazel hermetic archives, harness defaults, and the Linux release-factory contract with the same exact compiler release.
• Preserve the Rust 1.88 MSRV and the existing stripped cargo build --release git-overlay installation path.

Validation

• rustc -Vv and cargo -V select Rust 1.98.1 from rust-toolchain.toml.
• cargo fmt --all -- --check passes.
• cargo build --release --locked -p ctx succeeds with the optimized release profile.
• The Linux release-factory and SBOM contract tests pass, covering 36 tests.
• The Bazel Rust pin validator reports 5 release hosts, 1 source-only host, and 36 verified archives for the Rust 1.98.1 channel manifest; its 10 mutation tests pass.
• After the daemon handoff and restart, ctx status reports a running daemon and ready refresh/index state, and a ctx search --refresh off smoke query succeeds.

Fixes #970.