fix: apply PR #17 improvements with correct Rust toolchain version (1.85 → 1.86)

[Copilot]

Summary

This PR incorporates all the improvements from PR #17 while fixing the compilation error that caused CI to fail, and adds a Copilot reviewer instructions file.

Root Cause of PR #17 Failure

PR #17 added rust-toolchain.toml pinning Rust to 1.85, but cargo-bundle-licenses v4.2.0 pulls in:

• cargo-platform@0.3.1 — requires rustc ≥ 1.86
• cargo_metadata@0.23.0 — requires rustc ≥ 1.86

This caused the CI step cargo install --locked cargo-bundle-licenses to fail with:

error: failed to compile `cargo-bundle-licenses v4.2.0`
Caused by:
  rustc 1.85.1 is not supported by the following packages:
    cargo-platform@0.3.1 requires rustc 1.86
    cargo_metadata@0.23.0 requires rustc 1.86.0

Changes

File	Change
rust-toolchain.toml	Pin to 1.86 instead of 1.85 (fixes the compilation failure)
src/planner.rs	Use as_deref().unwrap_or() instead of .clone().unwrap_or_else() to avoid unnecessary heap allocation
.github/workflows/build-linux.yml	Add dtolnay/rust-toolchain@stable + Swatinem/rust-cache@v2 steps
.github/workflows/build-windows.yml	Same, plus remove redundant cargo test step (already covered by Invoke-Build Build,Test)
.gitignore	Add .DS_Store for macOS contributors
.github/copilot-review-instructions.md	New: instruct Copilot reviewer to verify all CI action runs pass before approving

Copilot Reviewer Instructions

Added .github/copilot-review-instructions.md with the following key instruction:

Verify that all GitHub Actions workflow runs triggered by the PR have completed successfully before approving or leaving a positive review. If any workflow run has failed or is still in progress, explicitly call this out and block approval until failures are resolved.

This would have caught the PR #17 failure automatically.