agent: P2-18c announce-and-approve enrolment path

When -enroll-server is supplied without -enroll-token, the agent
mints (and persists) an Ed25519 keypair, POSTs /api/agents/announce,
prints the SHA256 fingerprint in a copy-friendly banner, opens
/ws/agent/pending, signs the server's nonce, and blocks until the
admin clicks Accept (1h ceiling). On accept, persists the bearer +
host_id from the 'enrolled' message; on reject (close code 4001)
exits with a clear error.

Repo creds are pushed via config.update on the first standard WS
hello (P1-32 path), not in the enrolled message itself.

internal/agent/config/config.go

@@ -62,6 +62,13 @@ type Config struct {
 	LegacyRepoURL      string `yaml:"repo_url,omitempty"`
 	LegacyRepoPassword string `yaml:"repo_password,omitempty"`
 
+	// AnnounceKey is the base64-encoded Ed25519 private key used by
+	// announce-and-approve enrolment (P2-18). Generated on first
+	// announce, persisted so the agent can re-attach to the same
+	// pending row across restarts. 64 bytes when decoded.
+	// Empty for token-flow enrolments.
+	AnnounceKey string `yaml:"announce_key,omitempty"`
+
 	// path is the file we loaded from. Used by Save.
 	path string `yaml:"-"`
 }