Add copilot-setup-steps.yml for workspace agent environment

[Copilot]

Configures the GitHub Copilot coding agent environment for the yew workspace per the copilot-setup-steps spec.

Setup steps

• MSRV toolchain — 1.84.0 + wasm32-unknown-unknown target (matches CI matrix)
• Rust cache — Swatinem/rust-cache@v2 with save-if pattern (consistent with ci: improve rust-cache configuration for 100 GB cache limit #4038)
• Trunk — jetli/trunk-action@v0.5.1 (consistent with publish-examples / size-cmp workflows)
• typos — crate-ci/typos@v1.44.0 (consistent with main-checks workflow)
• cargo-hack — taiki-e/install-action@cargo-hack (consistent with clippy workflow per ci: replace hand-maintained feature-soundness scripts with cargo-hack #4013)
• wasm-bindgen-cli — via existing ci/install-wasm-bindgen-cli.sh (pins to version from Cargo.lock)
• cargo-make — primary dev task runner per CONTRIBUTING.md
• Permissions — scoped to contents: read

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[Madoshakalaka]

@copilot revisit this now that #4013 and #4038 are merged into master. rebase onto master.

https://raw.githubusercontent.com/Swatinem/rust-cache/refs/heads/master/README.md
has more information on the action configuration

add cargo-hack action to the agent setup too

[Copilot]

@copilot revisit this now that #4013 and #4038 are merged into master. rebase onto master.

https://raw.githubusercontent.com/Swatinem/rust-cache/refs/heads/master/README.md
...

Done in b028ea3. Merged master (with #4013 and #4038), updated rust-cache to use save-if pattern instead of shared-key, and added taiki-e/install-action@cargo-hack.

[Madoshakalaka]

solid

[Madoshakalaka]

I can't merge this by myself.

Merging is blocked
At least 1 approving review is required by reviewers with write access. Approvals from users that collaborated with the coding agent on changes will not satisfy review requirements.

It looks like GitHub's review policy is stricter for agents. (Good thing)

@WorldSEnder wanna review this?