Migrate CI to SciML centralized reusable workflows

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas

Normalizes CI to the SciML standard set via the centralized reusable workflows in SciML/.github (every call pinned @v1).

Workflows converted

• Tests.yml (was CI.yml) → tests.yml@v1. Preserves the exact matrix: version: ["1", "1.10"], ubuntu/x64 single OS, no test groups. Original collected coverage (codecov), so coverage is left at the default (enabled).
• Documentation.yml → documentation.yml@v1.

Workflows added (standard checks)

• FormatCheck.yml → runic.yml@v1. The repo did not previously run Runic, so Runic was installed and run in-place; the resulting formatting change to src/SciMLIterators.jl is included in this PR so the check passes.
• SpellCheck.yml → spellcheck.yml@v1. Ran typos locally — no findings, no _typos.toml needed.
• Downgrade.yml → downgrade.yml@v1 (julia-version: lts, skip: Pkg,TOML). None existed before.
• TagBot.yml — standard JuliaRegistries/TagBot@v1 setup. The package is registered in General and had no TagBot workflow.

Dependency automation

• Added .github/dependabot.yml: github-actions (weekly, /) and julia (daily, dirs / and /docs — the only dirs with a Project.toml; test/ has none) with the all-julia-packages group.
• CompatHelper: none existed; nothing to remove.

Notes / caveats

• The original CI.yml/Documentation.yml triggered on the master branch, but the repo's default branch is main. The new callers trigger on both master and main so they fire correctly on the actual default branch.
• Branch protection: check names change (e.g. Tests, Documentation, Format Check, Spell Check, Downgrade). Required-status-checks in branch protection must be updated accordingly.
• No downstream/IntegrationTest, benchmark, or QA workflows existed — none were invented.

🤖 Generated with Claude Code