server: drop in-process TLS — HTTP-only behind reverse proxy

Self-hosted deployments already terminate TLS at Caddy/Traefik/nginx;
making the server do TLS too means double cert config, dual ACME
plumbing, and an untested code path. Drop RM_TLS_CERT/RM_TLS_KEY,
remove TLSEnabled() and the ListenAndServeTLS branch.

Replace the cookie's "Secure if TLS-in-process" check with a new
RM_COOKIE_SECURE flag (default true). Local HTTP-only testing sets
RM_COOKIE_SECURE=false; production is always behind a TLS proxy and
the cookie stays Secure.

Default port :8443 → :8080. docker-compose binds 127.0.0.1 only and
populates RM_TRUSTED_PROXY. spec.md §4.1/§10.1 rewritten with a
Caddyfile snippet and a hard "do not expose RM_LISTEN publicly"
warning. enrollResponse keeps cert_pin_sha256 in the shape but the
server can't introspect a cert it doesn't terminate — operator
pastes the proxy's hash into -cert-pin at install time.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/server/http/enrollment.go

@@ -27,9 +27,12 @@ type enrollRequest struct {
 
 // enrollResponse hands the agent the credentials it'll use forever.
 // AgentToken is shown exactly once; the server stores its hash.
-// CertPinSHA256 is the SHA-256 of the server's certificate, base64;
-// the agent pins this on every reconnect so a stolen DB at the
-// control plane can't be replayed against an attacker's TLS endpoint.
+//
+// CertPinSHA256 is reserved for future use. The server is HTTP-only
+// and sits behind a reverse proxy that owns the TLS cert; pinning is
+// configured at the agent install step (`-cert-pin`) by the operator
+// pasting in the proxy's cert hash. The field stays in the response
+// shape so we can populate it later if the topology changes.
 type enrollResponse struct {
 	HostID        string `json:"host_id"`
 	AgentToken    string `json:"agent_token"`
@@ -109,9 +112,11 @@ func (s *Server) handleAgentEnroll(w stdhttp.ResponseWriter, r *stdhttp.Request)
 	writeJSON(w, stdhttp.StatusCreated, enrollResponse{
 		HostID:     hostID,
 		AgentToken: agentToken,
-		// CertPinSHA256 is populated by a TLS-aware future revision.
-		// For now (HTTP-or-TLS-by-Caddy) we leave it empty and rely
-		// on the agent trusting its OS root store.
+		// CertPinSHA256: the server is HTTP-only and sits behind a
+		// reverse proxy that owns the cert. The operator pastes the
+		// proxy's cert hash into the install command (`-cert-pin`)
+		// when they want pinning; the server cannot introspect a
+		// cert it doesn't terminate.
 	})
 }