SRE-1068: Centralize Rust toolchain updates in the Renovate preset

[TimDiekmann]

Rust toolchain updates should use one shared Renovate configuration and Rust's release manifests. This change groups the native rust-toolchain manager with the badge and mise lockfile updates, so all Rust toolchain pins receive the same nightly in one branch.

The preset removes the duplicate regex manager for rust-toolchain.toml, uses rust-version with rust-release-channel versioning for the badges, and takes over the Rust mise lockfile manager from hashintel/hash. The commit message uses the full nightly version returned by that datasource.

The companion change in hashintel/hash removes its local Rust toolchain manager and inherits these rules. Merge this preset change first, then the companion PR, before the next Rust update run.

Validation: Renovate 44.34.3 accepts both configurations. A local check using Renovate's extraction, version lookup, branch generation, and file replacement functions updated all five Rust pins to nightly-2026-09-05 on deps/rs/rust-toolchains. The generated commit message was Update Rust toolchains to nightly-2026-09-05. The 14-day minimum release age and existing schedule were preserved.

SRE-1068 (internal).

[cursor]

PR Summary

Low Risk
Renovate-only preset changes; no runtime app code, but incorrect grouping or datasources could desync Rust pins across repos until fixed.

Overview
Centralizes Rust nightly toolchain updates so Renovate’s native rust-toolchain manager, README badge pins, and .config/mise/mise.lock stay on one grouped Rust toolchains branch.

The shared package rule now matches custom.regex and rust-toolchain, and commit messages use to {{newVersion}} (full nightly from the rust-version datasource) instead of a hard-coded nightly- prefix.

Custom regex managers drop the rust-toolchain.toml duplicate and github-releases / rust-analyzer lookups in favor of rust-version with rust-release-channel versioning. Badge updates still scan README.md and src/lib.rs; a dedicated manager keeps the mise lockfile tools.rust version aligned when Rust is installed via the idiomatic toolchain file.

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