docs(skill): add local Rust Docker workflow

Document the pinned Rust image, reusable Cargo caches, PowerShell-safe bind mounts, targeted commands, base-worktree reproduction flow, and image limitations for local agent sessions.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: eisber

.github/skills/rust-docker/SKILL.md

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+---
+name: rust-docker
+description: Build, check, run, or test this Rust workspace in Docker when Cargo is unavailable on the local node. Use for local Rust validation and reproductions without installing a host toolchain.
+license: AGPL-3.0
+metadata:
+  author: eisber
+  version: "1.0"
+compatibility: Requires a running Docker daemon and PowerShell
+allowed-tools: Bash
+---
+
+# Rust validation with Docker
+
+Run Rust commands locally in the pinned `rust:1.91-bookworm` image. This matches
+the workspace's `rust-version = "1.91"` requirement and avoids modifying the
+host toolchain.
+
+## Prepare once
+
+From the repository root in PowerShell:
+
+```powershell
+docker info
+docker pull rust:1.91-bookworm
+docker volume create lox-cargo-registry
+docker volume create lox-cargo-target
+```
+
+The named volumes preserve downloaded dependencies and compiled artifacts
+between runs.
+
+## Run Cargo
+
+Resolve the repository path before mounting it:
+
+```powershell
+$repo = (Get-Location).Path
+docker run --rm `
+  --mount "type=bind,source=$repo,target=/work" `
+  --volume lox-cargo-registry:/usr/local/cargo/registry `
+  --volume lox-cargo-target:/work/target `
+  --workdir /work `
+  rust:1.91-bookworm `
+  cargo test --workspace --release
+```
+
+Replace the final Cargo arguments as needed:
+
+```text
+cargo fetch --locked
+cargo check --workspace
+cargo build --workspace --release
+cargo test test_name -- --nocapture
+cargo run -- <lox arguments>
+```
+
+Pass Cargo directly as the container command. Do not wrap it in `sh -lc` or
+`bash -lc`, because a login shell can replace the image's Rust `PATH`.
+
+## Run against a temporary base worktree
+
+For before/after regressions, create a detached worktree under the repository
+and mount it independently:
+
+```powershell
+git worktree add --detach .tmp-base origin/main
+$base = (Resolve-Path .tmp-base).Path
+docker run --rm `
+  --mount "type=bind,source=$base,target=/work" `
+  --volume lox-cargo-registry:/usr/local/cargo/registry `
+  --volume lox-cargo-target:/work/target `
+  --workdir /work `
+  rust:1.91-bookworm `
+  cargo test test_name -- --nocapture
+git worktree remove .tmp-base
+```
+
+Use a different target volume when base and head build artifacts must remain
+strictly isolated.
+
+## Limitations
+
+The pinned official image includes `cargo` and `rustc`, but not `rustfmt`,
+Clippy, or `rustup`. Use the repository CI or an image explicitly built with
+those components for `cargo fmt` and `cargo clippy`; do not report them as
+validated from this image.