feat(spec): add HostCommand and Refresh to CredentialSource

[zampani-docker]

Summary

Extends spec.CredentialSource with two new optional fields that enable credential helpers — shell commands that run on the host to obtain credential values:

• hostCommand — a POSIX sh -c command whose trimmed stdout becomes the credential value. Executed at sandbox creation time (CLI), on-demand by the daemon proxy, and periodically on the refresh cadence while sbx run is attached.

• refresh — a positive Go duration string (e.g. "15m", "1h") controlling how often the sandbox runtime re-runs the command and syncs the fresh token while the sandbox is attached. Requires hostCommand to be set.

Motivation

Some credentials live in platform-specific locations (Docker Desktop backend socket, vault read, aws sts get-session-token, gcloud auth print-access-token) and cannot be captured via env vars or static files. hostCommand provides a first-class credential helper interface for these cases.

Validation changes

ValidateCredentialPolicy is updated to:

• Accept hostCommand alone as a valid source (env, file, or hostCommand — at least one required)
• Reject blank/whitespace-only hostCommand
• Reject refresh without hostCommand
• Reject non-positive or unparseable refresh durations

The error message for empty sources changes from "must have at least one of env or file" to "must have at least one of env, file, or hostCommand".

Example spec.yaml usage

credentials:
  sources:
    my-service:
      hostCommand: |
        vault read -field=token secret/my-service
      refresh: 15m

Context

This unblocks a companion PR in the sandboxes engine that adds the full hostCommand credential source implementation. The strict YAML decoder (KnownFields(true)) in spec/artifact.go rejects any spec.yaml containing hostCommand: or refresh: until these fields are present — so merging here is a prerequisite for kit authors to use the feature.

Test plan

• go test ./spec/... — existing tests pass, 7 new subtests added for hostCommand/refresh validation

Comment drafted by Claude Code.