chore: delete rust-toolchain.toml from this TypeScript-only repo

[BryanFRD]

Problem

rust-toolchain.toml sits at the repo root:

[toolchain]
channel = "nightly-2026-08-03"
components = ["rustfmt", "clippy", "rust-src"]
profile = "minimal"

There is no Rust in this repo — six TypeScript packages, no Cargo.toml, no .rs file, no CI step that invokes cargo (.github/workflows/ci.yml calls the reusable Node workflow only).

Why it matters

Renovate tracks the file and opens a PR every time the nightly channel moves. Recent examples on main: ecaa12b (nightly-2026-08-03), e18c73b (nightly-2026-07-24), 64768d3 (nightly-2026-07-23), plus af38923 and 4672dba before them. Every one is a PR to review, a CI run to burn, and a commit on main that changes nothing, because nothing reads the file.

Proposed approach

• Delete rust-toolchain.toml.
• Confirm nothing in renovate.json pins a cargo/rust manager that would then error; drop the manager entry if one is configured.

Acceptance criteria

• The file is gone and no Renovate PR targets a Rust toolchain in this repo again.