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

kernel/include/starkernel/capsule_sig.h

@@ -22,6 +22,12 @@
 
 #include "starkernel/capsule.h"
 
+/* The certificate whose key the capsules are signed with: "StarshipOS
+ * Capsule Signing", issued by the StarshipOS Root CA (ci/pki/make-ca.sh;
+ * FABRIC-3.md XLVI), replacing the snakeoil intermediate of Milestone 6.
+ * mkcapsule refuses to sign with a key that is not this certificate's. */
+#define CAPSULE_SIGNING_CERT_NAME "pki:starshipos-intermediate.der"
+
 typedef enum {
     CAPSULE_SIG_OK = 0,       /* has_sig=1, and it verifies */
     CAPSULE_SIG_MISSING,      /* has_sig=0 -- not signed at all */
@@ -31,9 +37,9 @@ typedef enum {
 
 /*
  * Verify capsule descs[index]'s Ed25519 signature against the embedded
- * snakeoil intermediate cert's public key (capsule name
- * "pki:snakeoil-intermediate.der", found and parsed once, cached for
- * every later call this boot -- the cert doesn't change mid-boot).
+ * capsule-signing certificate's public key (CAPSULE_SIGNING_CERT_NAME,
+ * found and parsed once, cached for every later call this boot -- the
+ * cert doesn't change mid-boot).
  *
  * descs/names/sigs must be the same three parallel arrays
  * (capsule_get_descriptors()/capsule_get_names()/capsule_get_signatures()),

kernel/include/starkernel/x509_ed25519.h

@@ -6,9 +6,10 @@
  *
  * x509_extract_ed25519_pubkey() only ever reads SubjectPublicKeyInfo --
  * no signature verification, no chain validation, no extension parsing.
- * That's the capsule-PKI use case (Milestone 6): the embedded snakeoil
- * intermediate cert is trusted because it's baked into the trusted build,
- * never re-verified against the offline root CA at boot.
+ * That's the capsule-PKI use case (Milestone 6): the embedded capsule-
+ * signing cert (StarshipOS Capsule Signing since 2026-11-07, the snakeoil
+ * intermediate before it) is trusted because it's baked into the trusted
+ * build, never re-verified against its root CA at boot.
  *
  * x509_verify_signature()/x509_extract_serial() (added 2026-08-28,
  * FABRIC-2.md §F.7/§F.17) are for CERTVERIFY -- a regular user's cert,