fix(ci): drop beta-Rust CodeQL leg (cancelled/hanging 'checks 2 things forever') (#79)

## What

The CodeQL matrix analysed a `rust` leg. **CodeQL Rust support is beta**
— that leg cancels/hangs and never resolves (the recurring *"CodeQL
checks 2 things forever"* symptom), providing zero real security
coverage.

## Fix

Remove the `rust` leg; keep/ensure an `actions` leg so the matrix stays
non-empty (no zero-jobs `startup_failure`). Rust security belongs in
native tooling (cargo-audit / clippy), not beta CodeQL.

## Verification

`actionlint`: no parse/syntax errors; matrix has an `actions` leg and
zero active `rust` legs.

## Context

Estate-wide shared-fate: 13 repos carried this beta-Rust CodeQL leg.
Reference: hyperpolymath/gitbot-fleet#375.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/codeql.yml

@@ -27,8 +27,6 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - language: rust
-            build-mode: none
           - language: actions
             build-mode: none
     steps: