Centralize CI on SciML reusable workflows

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas.

Converts the CI to use the centralized SciML/.github reusable workflows (all pinned @v1, every caller gets secrets: "inherit").

Converted (inline -> central caller)

• FormatCheck.yml: fredrikekre/runic-action -> runic.yml@v1
• SpellCheck.yml: crate-ci/typos -> spellcheck.yml@v1
• Downgrade.yml: inline downgrade job -> downgrade.yml@v1, preserving the matrix exactly (group: Core, julia-version: 1.10, skip: Pkg,TOML; ALLOW_RERESOLVE: false is the caller default)

Already central (left as-is)

• Tests.yml: already a tests.yml@v1 caller (version × os matrix preserved). Retains its custom inline AllocCheck job (GROUP: nopre), which has no central equivalent.
• Documentation.yml: already a documentation.yml@v1 caller.

Cleanup

• dependabot.yml: removed the crate-ci/typos ignore block (spellcheck is now centralized); removed the /test julia directory entry (no Project.toml there). github-actions (weekly, /) and julia (daily, group all-julia-packages: ["*"] over /, /docs) blocks retained.
• No CompatHelper workflow present.
• TagBot.yml unchanged.

Checks

• Runic: ran Runic.main(["--inplace","."]) locally — no changes (repo was already Runic-clean from the prior inline check).
• Typos: typos . exits 0 — no fixes needed.

⚠️ Check names will change (e.g. the format/spellcheck/downgrade job names now come from the reusable workflows), so any branch-protection required-status-check names will need updating.

🤖 Generated with Claude Code