ci: extract per-job prelude (free-disk → checkout → bun → ui-build → apt → rust) into a composite action

[staging-devin-ai-integration]

Context

PR #434 added a Skit / Coverage job, which made the existing copy-paste prelude across Skit / Lint, Skit / Test, Skit / Coverage, and Skit / Build jobs visible:

- Free disk space (jlumbroso/free-disk-space)
- actions/checkout@v5
- oven-sh/setup-bun@v2 (bun 1.3.5)
- bun install + bun run build  (because streamkit-server embeds UI assets)
- apt install libvpx-dev nasm
- dtolnay/rust-toolchain@master (1.95.0 + components)
- Swatinem/rust-cache@v2

Same sequence appears in .github/workflows/skit.yml at:

• Lint job
• Test job
• Coverage job (new in ci: add code coverage via cargo-llvm-cov (backend) and Vitest v8 (UI) #434)
• Build job
• Test (GPU) job (slight variation — self-hosted runner skips some setup)

E2E workflow has overlapping but not identical prelude (it also boots the server).

Why this matters

• Bumping any tool version (Bun, Rust toolchain, libvpx, NASM, free-disk-space action) requires 4–5 identical edits across the file. Easy to miss one, and we've already shipped at least one mismatch from manual sync (different action SHAs on otherwise-identical steps).
• Adding a sixth job in the future (e.g. bench, fuzz, coverage-gpu) will be a copy-paste exercise rather than a one-line addition.

Proposed shape

Extract to .github/actions/setup-skit/action.yml as a composite action with inputs for the variations:

• rust-cache-key (different per job)
• rust-cache-workspaces (Coverage uses . -> target/coverage, others use default)
• rust-components (Coverage needs llvm-tools-preview)
• skip-ui-build (Lint job doesn't need it)
• runs-on-self-hosted (Test (GPU) skips the free-disk + apt steps)

Each job collapses to: uses: ./.github/actions/setup-skit + 1–4 inputs.

Out of scope

• Refactoring the UI workflow's prelude similarly (smaller, less duplication).
• Touching the E2E workflow — its prelude has more variation (runs the server, manages browser deps) and is its own thing.

Refs

• PR ci: add code coverage via cargo-llvm-cov (backend) and Vitest v8 (UI) #434: ci: add code coverage via cargo-llvm-cov (backend) and Vitest v8 (UI)
• Review comment that surfaced this: ci: add code coverage via cargo-llvm-cov (backend) and Vitest v8 (UI) #434