ci(release): tag-driven release with attested binaries and crates.io trusted publishing (LAB-5117)

[27Bslash6]

Adds .github/workflows/release.yml and an install section in the README.

A version tag (v0.1.0, v0.2.0-rc.1) runs four jobs, all on GitHub-hosted runners:

1. check: the tag must equal v plus the crate version, the tagged commit must be on main, and cargo package --locked must build.
2. build: static musl binaries for x86_64 and aarch64 Linux and a binary for Apple silicon. Each job runs the test suite on its own target before it builds. These jobs hold only contents: read, so dependency build scripts never see a signing or publishing token.
3. release: attests each archive and the ck binary inside it with actions/attest (the action attest-build-provenance now wraps), then creates the GitHub Release. A tag with a - makes a pre-release.
4. publish: exchanges the job's OIDC token for a short-lived crates.io token (trusted publishing, release environment) and runs cargo publish. It runs last because a crates.io version can never be republished.

No installer script ships. The README documents cargo install --locked cachekit-cli and the pinned verify command, and says why --owner cachekit-io alone is not enough.

Before the first tag, the crate must exist on crates.io with this workflow registered as its trusted publisher, and the repository must be public, because artifact attestations need a public repository on this plan.

Closes LAB-5117

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Kody Code Review — 2 suggested fixes.
Paste the prompt below to your agent and all review fixed at once!

🛠️ Open Agent Prompt

A code review identified the following issues in this pull request.
Each section describes what was found and includes a reference implementation where available.

Files involved:
- .github/workflows/release.yml:41
- .github/workflows/release.yml:153

---

### [1/2] .github/workflows/release.yml:41
Issue identified during code review:
Trust-boundary bypass in the "Tagged commit is on main" step: the main-ancestry check runs from whatever release.yml exists at the tagged commit, so the workflow it protects can remove it. When anyone with tag-push rights tags an unreviewed commit whose release.yml drops this step, the `release` job still runs with contents/attestations/id-token write and no environment gate and signs as `.github/workflows/release.yml@refs/tags/vX`. The README verification command (`--repo`, path-only `--signer-workflow`, `--deny-self-hosted-runners`) accepts that attestation, which breaks the guarantee that only reviewed code on main is released. Fix: restrict `v*` tag creation with a repository tag ruleset and gate the `release` job on a protected `release` environment with deployment rules limited to `v*` tags, optionally having users pin `--signer-digest` or `--source-digest`.
Reference implementation (from code review):

// .github/workflows/release.yml:41
release:
    needs: build
    runs-on: ubuntu-latest
    timeout-minutes: 15
    # Gate signing on a protected environment (deployment rules: tags v*),
    # and protect v* tag creation with a repository ruleset, because the
    # main-ancestry check above runs from the tagged commit's own workflow file.
    environment: release
    permissions:
      contents: write
      id-token: write
      attestations: write

---

### [2/2] .github/workflows/release.yml:153
Issue identified during code review:
Bootstrap failure in the `publish` job: `rust-lang/crates-io-auth-action` requires a trusted publisher, which crates.io only allows on an existing crate, and this crate is unpublished (Cargo.toml is at 0.0.0, and the replaced README states nothing is published yet). When `v0.1.0` is pushed, the `release` job creates the GitHub Release and then the auth step in `publish` fails, so the version exists on GitHub but not on crates.io and the README's `cargo install --locked cachekit-cli` fails. Fix: before tagging, publish the first version (or a placeholder) manually with an API token and configure the trusted publisher, or document this bootstrap step next to the `publish` job.
Reference implementation (from code review):

// .github/workflows/release.yml:153
# Prerequisite: the crate must already exist on crates.io and have this
      # workflow + `release` environment registered as a trusted publisher.
      # Publish the first version manually with an API token before tagging.
      - uses: rust-lang/crates-io-auth-action@c6f97d42243bad5fab37ca0427f495c86d5b1a18 # v1.0.5
        id: auth

---

Review each issue in context, use the reference implementations as guidance, and apply fixes that are consistent with the surrounding codebase.

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• Attach a repo file as context: use an explicit marker like `@file:docs/guide.mdx#L10-L50` (replace with your real path).
• API Contract Docs: include `@file:openapi.yaml` or `@file:swagger.json` when changing routes/schemas.
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• Design System Source of Truth: include `@file:ui/index.ts` (replace with your DS entrypoint path).
• Feature Flags: include the flag key/name and `@file:flags.ts` / `@file:config.json` (and optionally the PostHog flag name).
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