pki: the StarshipOS CA, made on the CI box; capsules signed in CI

Ruled by Captain Bob, 2026-11-07: "New root CA on the CI box. Valid
StarshipOS rather than snake oil intermediate."

- ci/pki/make-ca.sh makes the StarshipOS Root CA (its key stays on the CI
  box) and the StarshipOS Capsule Signing intermediate it issues, and
  stores the signing key as the secret CAPSULE_SIGN_KEY.
- The kernel finds the signing certificate by one name,
  CAPSULE_SIGNING_CERT_NAME = pki:starshipos-intermediate.der. The
  certificate itself lands once make-ca.sh has run; until then the
  snakeoil intermediate is in the tree under its old name and the
  capsules check as "no root key" (a warning, as "missing" was).
- mkcapsule refuses a signing key that is not that certificate's.
- SIGN_KEY has no default; acceptance, build, nightly and release set it
  from the secret.

FABRIC-3.md XLVI.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GE3SDvQn5EoxdGufk84zz2

kernel/include/starkernel/capsule.h

@@ -171,7 +171,8 @@ typedef struct {
  *
  * Milestone 6 (Phase 8): each capsule's payload bytes (the same bytes
  * content_hash already covers), signed by mkcapsule at build time with
- * the snakeoil intermediate's private key. has_sig=0 for a capsule built
+ * the capsule-signing key (StarshipOS Capsule Signing, capsule_sig.h;
+ * the snakeoil intermediate's until 2026-11-07). has_sig=0 for a capsule built
  * before this feature existed or otherwise unsigned -- a real, distinct
  * state, not "signature is all-zero bytes" (which sig[64] full of 0x00
  * would otherwise look ambiguous with). Indexed 1:1 with