feat(security): security hardening, proxy support, and GL045 HTTP include warning

Security fixes:
- Path traversal guard in include: local: — paths with ../ that escape
  the repo root are rejected instead of reading arbitrary host files
- HTTP timeout (30 s) on all fetcher requests to prevent indefinite hangs
- Response size cap (10 MiB) via io.LimitReader to prevent memory exhaustion
- Cache directory and file permissions tightened to 0700/0600
- LSP Content-Length cap (64 MiB) to guard against DoS from a malicious client

New feature:
- --proxy flag on check, graph, and lsp subcommands; also proxy: key in
  .glint.yml; overrides system HTTP_PROXY / HTTPS_PROXY env vars when set;
  cmdGraph and cmdLSP now also load .glint.yml for proxy/token/url fallbacks

New lint rule:
- GL045 (Warning): include: remote: using plain http:// instead of https://

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/lsp/server_test.go

@@ -364,6 +364,20 @@ func TestServer_DidClose_ClearsdiAgnostics(t *testing.T) {
 	}
 }
 
+func TestServer_ContentLengthTooLarge(t *testing.T) {
+	// Craft a header with a content-length that exceeds the cap.
+	// The server must reject it before allocating a giant buffer.
+	header := fmt.Sprintf("Content-Length: %d\r\n\r\n", maxLSPMessageBytes+1)
+	srv, _, _ := newTestServer([]byte(header))
+	err := srv.Run()
+	if err == nil {
+		t.Fatal("expected error for oversized Content-Length, got nil")
+	}
+	if !strings.Contains(err.Error(), "exceeds maximum") {
+		t.Errorf("unexpected error: %v", err)
+	}
+}
+
 func TestServer_UriToPath(t *testing.T) {
 	tests := []struct {
 		uri  string