fix(agent): point Phase B fail-fast guard at SPEC-018

The SPEC-016 Phase B credential-store guard referenced "SPEC-017" for the
forthcoming SYSTEM service host, but 017 is now Mike's end-user-access
spec; the service host is SPEC-018. Comment + error-string text only, no
logic change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

agent/src/credential_store.rs

@@ -146,7 +146,7 @@ pub fn store_cak(cak: &str) -> Result<()> {
             "[ENROLL] wrote the credential store but cannot read it back in THIS security \
              context (access denied). The store is ACL'd to SYSTEM + Administrators by \
              design; the managed agent must run as the GuruConnect SYSTEM service (see \
-             SPEC-017) to read it. Refusing to leave an unreadable store behind."
+             SPEC-018) to read it. Refusing to leave an unreadable store behind."
         )),
         Err(e) => Err(anyhow::Error::new(e)
             .context("stored the credential but the immediate read-back verification failed")),

agent/src/main.rs

@@ -373,7 +373,7 @@ async fn resolve_agent_credential(config: &mut config::Config) -> Result<()> {
             // for the target (Option A) and is deliberately kept.
             //
             // NOTE: this guard is satisfied/removed once the GuruConnect SYSTEM
-            // service host lands (separate spec, SPEC-017) and the agent always
+            // service host lands (separate spec, SPEC-018) and the agent always
             // runs as SYSTEM — at which point the store is always readable.
             Err(LoadCakError::Io {
                 permission_denied: true,
@@ -382,7 +382,7 @@ async fn resolve_agent_credential(config: &mut config::Config) -> Result<()> {
                 return Err(anyhow::anyhow!(
                     "[ENROLL] credential store is not accessible in this context \
                      ({source}) — the managed agent must run as the GuruConnect SYSTEM \
-                     service (see SPEC-017). Refusing to re-enroll."
+                     service (see SPEC-018). Refusing to re-enroll."
                 ));
             }
             // M1 — other IO error reaching the store (not access-denied): also