Add OctoSTS org trust policy for toolhive-doc-mcp refresh

[Nashon-Steffen]

Summary

• Add .github/chainguard/toolhive-doc-refresh.sts.yaml. This is the first org-level OctoSTS trust policy.
• Add a CODEOWNERS entry for .github/chainguard/.

The toolhive-doc-mcp refresh CronJob reads docs from three public repos. It uses a token only for the GitHub API rate limit. The old classic PAT is expired, and every nightly refresh fails with 401 Bad credentials. The CronJob now exchanges an EKS service account token for a GitHub App token.

The policy allows exactly one subject, system:serviceaccount:toolhive-doc-mcp:toolhive-doc-refresh. It accepts that subject only from the dev, staging, and production EKS OIDC issuers. The token covers toolhive, toolhive-studio, and toolhive-catalog with contents: read only.

The infra side is stacklok/infra (link in the first comment).

Prerequisites (org admin)

• Install stacklok-octosts on stacklok/.github. OctoSTS reads org policies from this repo.
• Install stacklok-octosts on toolhive, toolhive-studio, and toolhive-catalog. GitHub rejects the token request if the policy lists a repo outside the installation.

Security note

With the app installed here, a policy in this repo can mint tokens for any repo where the app is installed. Those tokens can carry up to the app's own permissions. The main branch protection requires a Code Owner review, so the new /.github/chainguard/ entry makes @stacklok/maintain approval mandatory for every policy change.

Validation

• An EKS token from staging reached the policy lookup at octo-sts.stacklok.dev and returned unable to find trust policy. The issuer and audience checks pass today.
• The issuer_pattern matches the staging issuer as a full anchored match.
• After merge, the infra PR runs a manual staging Job to confirm the real token mint.

🤖 Generated with Claude Code

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[Nashon-Steffen]

Infra side: https://github.com/stacklok/infra/pull/5897